<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019c540a-9cb0-7c9d-b698-310bbc047c77,g:8a68b5e8-d9bc-4057-b1f6-ead794f175db-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.nationalmi.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYUNAUDITED" xlink:type="simple" xlink:href="nmih-20260331.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYUNAUDITED"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYUNAUDITED" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019c540a-9e2a-769c-906e-133a92d3ec2e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019c540a-9e2a-7b61-91f9-6cb892920f73" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019c540a-9e2a-769c-906e-133a92d3ec2e" xlink:to="loc_us-gaap_StatementTable_019c540a-9e2a-7b61-91f9-6cb892920f73" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019c540a-9e2a-7b0d-ba76-1aab7e79e446" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019c540a-9e2a-7b61-91f9-6cb892920f73" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019c540a-9e2a-7b0d-ba76-1aab7e79e446" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019c540a-9e2a-7b0d-ba76-1aab7e79e446_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019c540a-9e2a-7b0d-ba76-1aab7e79e446" xlink:to="loc_us-gaap_EquityComponentDomain_019c540a-9e2a-7b0d-ba76-1aab7e79e446_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019c540a-9e2a-7cd3-87b1-2a5df5150da1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019c540a-9e2a-7b0d-ba76-1aab7e79e446" xlink:to="loc_us-gaap_EquityComponentDomain_019c540a-9e2a-7cd3-87b1-2a5df5150da1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019c540a-9e2a-731e-9dd7-9f7fb85f4238" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c540a-9e2a-7cd3-87b1-2a5df5150da1" xlink:to="loc_us-gaap_CommonStockMember_019c540a-9e2a-731e-9dd7-9f7fb85f4238" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019c540a-9e2a-7bfc-b033-03767ce5769a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c540a-9e2a-7cd3-87b1-2a5df5150da1" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019c540a-9e2a-7bfc-b033-03767ce5769a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_019c540a-9e2a-7eae-b329-ee5ae7022fe2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c540a-9e2a-7cd3-87b1-2a5df5150da1" xlink:to="loc_us-gaap_TreasuryStockCommonMember_019c540a-9e2a-7eae-b329-ee5ae7022fe2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019c540a-9e2a-7b4b-8b21-197fb1660772" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c540a-9e2a-7cd3-87b1-2a5df5150da1" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019c540a-9e2a-7b4b-8b21-197fb1660772" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019c540a-9e2a-70d8-b0c3-8a9687bf8385" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c540a-9e2a-7cd3-87b1-2a5df5150da1" xlink:to="loc_us-gaap_RetainedEarningsMember_019c540a-9e2a-70d8-b0c3-8a9687bf8385" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c540a-9e2a-7d4a-ba3d-e0dae3b7b328" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c540a-9e2a-769c-906e-133a92d3ec2e" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c540a-9e2a-7d4a-ba3d-e0dae3b7b328" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019c540a-9e2a-7e53-a5d0-4006e36d39b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c540a-9e2a-7d4a-ba3d-e0dae3b7b328" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019c540a-9e2a-7e53-a5d0-4006e36d39b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019c540a-9e2a-7e7e-8eb1-476190f8c045" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c540a-9e2a-7d4a-ba3d-e0dae3b7b328" xlink:to="loc_us-gaap_StockholdersEquity_019c540a-9e2a-7e7e-8eb1-476190f8c045" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlansNetofSharesForTaxWithholdings_019c540a-9e2a-7db7-a352-8a7b359f2e6a" xlink:href="nmih-20260331.xsd#nmih_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlansNetofSharesForTaxWithholdings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c540a-9e2a-7d4a-ba3d-e0dae3b7b328" xlink:to="loc_nmih_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlansNetofSharesForTaxWithholdings_019c540a-9e2a-7db7-a352-8a7b359f2e6a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_StockIssuedDuringPeriodValueEmployeeStockPurchasePlanNetofTaxWithholdings_019c540a-9e2a-7ae9-8e8e-fccdf5cfcfe2" xlink:href="nmih-20260331.xsd#nmih_StockIssuedDuringPeriodValueEmployeeStockPurchasePlanNetofTaxWithholdings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c540a-9e2a-7d4a-ba3d-e0dae3b7b328" xlink:to="loc_nmih_StockIssuedDuringPeriodValueEmployeeStockPurchasePlanNetofTaxWithholdings_019c540a-9e2a-7ae9-8e8e-fccdf5cfcfe2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019c540a-9e2a-796b-9639-9265fcddf897" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c540a-9e2a-7d4a-ba3d-e0dae3b7b328" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019c540a-9e2a-796b-9639-9265fcddf897" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_019c540a-9e2a-7e97-9cd0-02b999d16bc5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c540a-9e2a-7d4a-ba3d-e0dae3b7b328" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_019c540a-9e2a-7e97-9cd0-02b999d16bc5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019c540a-9e2a-747c-8b99-7e2249db5bda" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c540a-9e2a-7d4a-ba3d-e0dae3b7b328" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019c540a-9e2a-747c-8b99-7e2249db5bda" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019c540a-9e2a-748a-ac1e-71d6d2932954" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c540a-9e2a-7d4a-ba3d-e0dae3b7b328" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019c540a-9e2a-748a-ac1e-71d6d2932954" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019c540a-9e2a-72b9-8176-ae6b82c58183" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c540a-9e2a-7d4a-ba3d-e0dae3b7b328" xlink:to="loc_us-gaap_NetIncomeLoss_019c540a-9e2a-72b9-8176-ae6b82c58183" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019c540a-9e2a-7dd4-bd9c-9eebeb039d94" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019c540a-9e2a-79f0-833b-51dc6bebfd8d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/InvestmentsScheduleofFairValuesandGrossUnrealizedGainsandLossesDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#InvestmentsScheduleofFairValuesandGrossUnrealizedGainsandLossesDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/InvestmentsScheduleofFairValuesandGrossUnrealizedGainsandLossesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7393-a2a3-36ef514ae5d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c540a-9e2a-767e-a2de-484499605f9a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7393-a2a3-36ef514ae5d0" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c540a-9e2a-767e-a2de-484499605f9a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-7063-b4e8-8c2ba9046b35" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c540a-9e2a-767e-a2de-484499605f9a" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-7063-b4e8-8c2ba9046b35" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7063-b4e8-8c2ba9046b35_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-7063-b4e8-8c2ba9046b35" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7063-b4e8-8c2ba9046b35_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7389-ac3c-c7fc0ae52397" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-7063-b4e8-8c2ba9046b35" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7389-ac3c-c7fc0ae52397" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_019c540a-9e2a-7f32-9252-d581a25a5e40" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7389-ac3c-c7fc0ae52397" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_019c540a-9e2a-7f32-9252-d581a25a5e40" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019c540a-9e2a-7080-a22a-5ad788e8198f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7389-ac3c-c7fc0ae52397" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019c540a-9e2a-7080-a22a-5ad788e8198f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019c540a-9e2a-7619-aae4-f24c6f29e557" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7389-ac3c-c7fc0ae52397" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019c540a-9e2a-7619-aae4-f24c6f29e557" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cf6ae-56e3-73f8-bfce-3f8e68fc717c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7389-ac3c-c7fc0ae52397" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cf6ae-56e3-73f8-bfce-3f8e68fc717c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_OtherAssetBackedSecuritiesMember_019d951b-1273-7fba-9896-7bf77e6b36cb" xlink:href="nmih-20260331.xsd#nmih_OtherAssetBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7389-ac3c-c7fc0ae52397" xlink:to="loc_nmih_OtherAssetBackedSecuritiesMember_019d951b-1273-7fba-9896-7bf77e6b36cb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BondsMember_019c540a-9e2a-7874-8416-50d4cf80d197" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7389-ac3c-c7fc0ae52397" xlink:to="loc_us-gaap_BondsMember_019c540a-9e2a-7874-8416-50d4cf80d197" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermInvestmentsMember_019c540a-9e2a-7164-b38e-d7e9ecb252c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7389-ac3c-c7fc0ae52397" xlink:to="loc_us-gaap_ShortTermInvestmentsMember_019c540a-9e2a-7164-b38e-d7e9ecb252c8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019c540a-9e2a-7a7b-80ee-e419684ff38b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7393-a2a3-36ef514ae5d0" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019c540a-9e2a-7a7b-80ee-e419684ff38b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019c540a-9e2a-77d4-a85a-89de7157c004" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7393-a2a3-36ef514ae5d0" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019c540a-9e2a-77d4-a85a-89de7157c004" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019c540a-9e2a-766f-b009-f5f267efbb64" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7393-a2a3-36ef514ae5d0" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019c540a-9e2a-766f-b009-f5f267efbb64" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019c540a-9e2a-7e3e-b76c-970bcbf86366" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7393-a2a3-36ef514ae5d0" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019c540a-9e2a-7e3e-b76c-970bcbf86366" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/InvestmentsScheduleofCorporateDebtSecuritiesbyIndustryGroupDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#InvestmentsScheduleofCorporateDebtSecuritiesbyIndustryGroupDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/InvestmentsScheduleofCorporateDebtSecuritiesbyIndustryGroupDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7e2a-b36a-2b7e1a3779fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c540a-9e2a-72e2-8543-b2774be5225a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7e2a-b36a-2b7e1a3779fa" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c540a-9e2a-72e2-8543-b2774be5225a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesByIndustryAxis_019c540a-9e2a-79fb-8f20-daf2df207948" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesByIndustryAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c540a-9e2a-72e2-8543-b2774be5225a" xlink:to="loc_us-gaap_EquitySecuritiesByIndustryAxis_019c540a-9e2a-79fb-8f20-daf2df207948" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019c540a-9e2a-79fb-8f20-daf2df207948_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019c540a-9e2a-79fb-8f20-daf2df207948" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019c540a-9e2a-79fb-8f20-daf2df207948_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019c540a-9e2a-719c-8d9c-5cf97629b0a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019c540a-9e2a-79fb-8f20-daf2df207948" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019c540a-9e2a-719c-8d9c-5cf97629b0a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_FinancialIndustryGroupMember_019c540a-9e2a-721b-968b-7db678324bd2" xlink:href="nmih-20260331.xsd#nmih_FinancialIndustryGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019c540a-9e2a-719c-8d9c-5cf97629b0a7" xlink:to="loc_nmih_FinancialIndustryGroupMember_019c540a-9e2a-721b-968b-7db678324bd2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ConsumerIndustryGroupMember_019c540a-9e2a-774c-9cc1-28569956e82b" xlink:href="nmih-20260331.xsd#nmih_ConsumerIndustryGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019c540a-9e2a-719c-8d9c-5cf97629b0a7" xlink:to="loc_nmih_ConsumerIndustryGroupMember_019c540a-9e2a-774c-9cc1-28569956e82b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_UtilitiesIndustryGroupMember_019c540a-9e2a-7ddc-a02b-ae0d1507b42c" xlink:href="nmih-20260331.xsd#nmih_UtilitiesIndustryGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019c540a-9e2a-719c-8d9c-5cf97629b0a7" xlink:to="loc_nmih_UtilitiesIndustryGroupMember_019c540a-9e2a-7ddc-a02b-ae0d1507b42c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_IndustrialIndustryGroupMember_019c540a-9e2a-7840-9916-d7e01ff56f9d" xlink:href="nmih-20260331.xsd#nmih_IndustrialIndustryGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019c540a-9e2a-719c-8d9c-5cf97629b0a7" xlink:to="loc_nmih_IndustrialIndustryGroupMember_019c540a-9e2a-7840-9916-d7e01ff56f9d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_TechnologyIndustryGroupMember_019c540a-9e2a-76b0-a87e-657f1b2a0850" xlink:href="nmih-20260331.xsd#nmih_TechnologyIndustryGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019c540a-9e2a-719c-8d9c-5cf97629b0a7" xlink:to="loc_nmih_TechnologyIndustryGroupMember_019c540a-9e2a-76b0-a87e-657f1b2a0850" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_CommunicationsIndustryGroupMember_019c540a-9e2a-7d6c-a905-411215530bd1" xlink:href="nmih-20260331.xsd#nmih_CommunicationsIndustryGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019c540a-9e2a-719c-8d9c-5cf97629b0a7" xlink:to="loc_nmih_CommunicationsIndustryGroupMember_019c540a-9e2a-7d6c-a905-411215530bd1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_BasicMaterialsMember_019c540a-9e2a-79bb-b438-eb4e61339d7a" xlink:href="nmih-20260331.xsd#nmih_BasicMaterialsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019c540a-9e2a-719c-8d9c-5cf97629b0a7" xlink:to="loc_nmih_BasicMaterialsMember_019c540a-9e2a-79bb-b438-eb4e61339d7a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_DebtSecuritiesCorporateComponentOfTotalPercentage_019c540a-9e2a-7521-8e51-5375d9c49909" xlink:href="nmih-20260331.xsd#nmih_DebtSecuritiesCorporateComponentOfTotalPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7e2a-b36a-2b7e1a3779fa" xlink:to="loc_nmih_DebtSecuritiesCorporateComponentOfTotalPercentage_019c540a-9e2a-7521-8e51-5375d9c49909" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/InvestmentsNarrativeDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#InvestmentsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/InvestmentsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-72d2-b164-083488dafd08" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c540a-9e2a-75a9-890d-eb28b0d83f0d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-72d2-b164-083488dafd08" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c540a-9e2a-75a9-890d-eb28b0d83f0d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-7c07-90b4-e0921739054f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c540a-9e2a-75a9-890d-eb28b0d83f0d" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-7c07-90b4-e0921739054f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7c07-90b4-e0921739054f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-7c07-90b4-e0921739054f" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7c07-90b4-e0921739054f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-767c-8e42-44917e65cdd4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-7c07-90b4-e0921739054f" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-767c-8e42-44917e65cdd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_019c540a-9e2a-77a8-b49c-4a21d4364654" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-767c-8e42-44917e65cdd4" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_019c540a-9e2a-77a8-b49c-4a21d4364654" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsAndShortTermInvestments_019c540a-9e2a-743f-932e-7940708d39a3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsAndShortTermInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-72d2-b164-083488dafd08" xlink:to="loc_us-gaap_CashCashEquivalentsAndShortTermInvestments_019c540a-9e2a-743f-932e-7940708d39a3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_PayableForUnsettledTradeTransactions_019d95d2-1762-73ed-b927-223b8d32e1f1" xlink:href="nmih-20260331.xsd#nmih_PayableForUnsettledTradeTransactions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-72d2-b164-083488dafd08" xlink:to="loc_nmih_PayableForUnsettledTradeTransactions_019d95d2-1762-73ed-b927-223b8d32e1f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReceivableForUnsettledTradeTransactions_019dd663-af08-7562-8386-13d06901b865" xlink:href="nmih-20260331.xsd#nmih_ReceivableForUnsettledTradeTransactions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-72d2-b164-083488dafd08" xlink:to="loc_nmih_ReceivableForUnsettledTradeTransactions_019dd663-af08-7562-8386-13d06901b865" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019c540a-9e2a-7546-b749-95d34f2465b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-72d2-b164-083488dafd08" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019c540a-9e2a-7546-b749-95d34f2465b3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019c540a-9e2a-71f4-b65b-47f43cd3adcc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-72d2-b164-083488dafd08" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019c540a-9e2a-71f4-b65b-47f43cd3adcc" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/InvestmentsScheduleofInvestmentsbyMaturityDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#InvestmentsScheduleofInvestmentsbyMaturityDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/InvestmentsScheduleofInvestmentsbyMaturityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019cf6ba-1b0a-7140-a1e3-f98b1a85d641" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019cf6ba-1b0a-779c-a048-a9425a7d2d0e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019cf6ba-1b0a-7140-a1e3-f98b1a85d641" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019cf6ba-1b0a-779c-a048-a9425a7d2d0e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019cf6ba-1b0a-748c-b083-c24e0c7b40e2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019cf6ba-1b0a-779c-a048-a9425a7d2d0e" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019cf6ba-1b0a-748c-b083-c24e0c7b40e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cf6ba-1b0a-748c-b083-c24e0c7b40e2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019cf6ba-1b0a-748c-b083-c24e0c7b40e2" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cf6ba-1b0a-748c-b083-c24e0c7b40e2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cf6ba-1b0a-7f65-b3a6-5574ac33af2a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019cf6ba-1b0a-748c-b083-c24e0c7b40e2" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cf6ba-1b0a-7f65-b3a6-5574ac33af2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_AssetBackedSecurityNetOfMortgageBackedSecurityIssuedByUSGovernmentSponsoredEnterpriseMember_019cf6ba-1b0a-7caf-af83-f07a79e70d30" xlink:href="nmih-20260331.xsd#nmih_AssetBackedSecurityNetOfMortgageBackedSecurityIssuedByUSGovernmentSponsoredEnterpriseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cf6ba-1b0a-7f65-b3a6-5574ac33af2a" xlink:to="loc_nmih_AssetBackedSecurityNetOfMortgageBackedSecurityIssuedByUSGovernmentSponsoredEnterpriseMember_019cf6ba-1b0a-7caf-af83-f07a79e70d30" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cf6ba-1b0a-79e3-b12f-f28e1a517d2a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019cf6ba-1b0a-7f65-b3a6-5574ac33af2a" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cf6ba-1b0a-79e3-b12f-f28e1a517d2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019c540a-9e2a-7433-8e8e-a5db4dfe0286" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019cf6ba-1b0a-7140-a1e3-f98b1a85d641" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019c540a-9e2a-7433-8e8e-a5db4dfe0286" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsAmortizedCostBasis_019c540a-9e2a-7f4a-93a2-7d5d35ef5b32" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019c540a-9e2a-7433-8e8e-a5db4dfe0286" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsAmortizedCostBasis_019c540a-9e2a-7f4a-93a2-7d5d35ef5b32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis_019c540a-9e2a-7758-a1d9-2640c1a592da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019c540a-9e2a-7433-8e8e-a5db4dfe0286" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis_019c540a-9e2a-7758-a1d9-2640c1a592da" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenAmortizedCostBasis_019c540a-9e2a-700f-bef2-97cbffddb5dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019c540a-9e2a-7433-8e8e-a5db4dfe0286" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenAmortizedCostBasis_019c540a-9e2a-700f-bef2-97cbffddb5dc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenAmortizedCostBasis_019c540a-9e2a-7f8d-b01f-ce05d684cbe1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019c540a-9e2a-7433-8e8e-a5db4dfe0286" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenAmortizedCostBasis_019c540a-9e2a-7f8d-b01f-ce05d684cbe1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost_019c540a-9e2a-74fb-80be-5d85d6d3224b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019c540a-9e2a-7433-8e8e-a5db4dfe0286" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost_019c540a-9e2a-74fb-80be-5d85d6d3224b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019c540a-9e2a-79e1-857d-82f7c259373b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019c540a-9e2a-7433-8e8e-a5db4dfe0286" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019c540a-9e2a-79e1-857d-82f7c259373b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019c540a-9e2a-7c02-92e9-df346a62db3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019cf6ba-1b0a-7140-a1e3-f98b1a85d641" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019c540a-9e2a-7c02-92e9-df346a62db3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue_019c540a-9e2a-79d0-a922-6a171385ddb8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019c540a-9e2a-7c02-92e9-df346a62db3c" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesNextRollingTwelveMonthsFairValue_019c540a-9e2a-79d0-a922-6a171385ddb8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue_019c540a-9e2a-717e-a38d-92271c8c2cd8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019c540a-9e2a-7c02-92e9-df346a62db3c" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearTwoThroughFiveFairValue_019c540a-9e2a-717e-a38d-92271c8c2cd8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue_019c540a-9e2a-7e57-a06b-853440e0747c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019c540a-9e2a-7c02-92e9-df346a62db3c" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingYearSixThroughTenFairValue_019c540a-9e2a-7e57-a06b-853440e0747c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenFairValue_019c540a-9e2a-7b4c-a012-2b4927f3c455" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019c540a-9e2a-7c02-92e9-df346a62db3c" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesRollingAfterYearTenFairValue_019c540a-9e2a-7b4c-a012-2b4927f3c455" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue_019c540a-9e2a-73e3-818c-78fd80c7cc93" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019c540a-9e2a-7c02-92e9-df346a62db3c" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue_019c540a-9e2a-73e3-818c-78fd80c7cc93" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019c540a-9e2a-7640-bac8-e574120ce278" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAbstract_019c540a-9e2a-7c02-92e9-df346a62db3c" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019c540a-9e2a-7640-bac8-e574120ce278" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/InvestmentsScheduleofAgingofUnrealizedLossesDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#InvestmentsScheduleofAgingofUnrealizedLossesDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/InvestmentsScheduleofAgingofUnrealizedLossesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7193-8737-3df769d9fe5e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c540a-9e2a-7d03-9780-91f3ed1ab26a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7193-8737-3df769d9fe5e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c540a-9e2a-7d03-9780-91f3ed1ab26a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-74e0-b15b-a59aa1e76df2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c540a-9e2a-7d03-9780-91f3ed1ab26a" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-74e0-b15b-a59aa1e76df2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-74e0-b15b-a59aa1e76df2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-74e0-b15b-a59aa1e76df2" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-74e0-b15b-a59aa1e76df2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-74a8-813b-7d858e6ec637" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-74e0-b15b-a59aa1e76df2" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-74a8-813b-7d858e6ec637" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_019c540a-9e2a-75f4-8415-32b9f05c054c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-74a8-813b-7d858e6ec637" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_019c540a-9e2a-75f4-8415-32b9f05c054c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019c540a-9e2a-795e-b803-837b99232cbd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-74a8-813b-7d858e6ec637" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019c540a-9e2a-795e-b803-837b99232cbd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019c540a-9e2a-72e8-9e66-d8fb64066abb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-74a8-813b-7d858e6ec637" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019c540a-9e2a-72e8-9e66-d8fb64066abb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cf6bc-4034-7819-990d-bd7aed071dca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-74a8-813b-7d858e6ec637" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cf6bc-4034-7819-990d-bd7aed071dca" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_OtherAssetBackedSecuritiesMember_019d951c-8312-742d-9f8a-69d889605e1c" xlink:href="nmih-20260331.xsd#nmih_OtherAssetBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-74a8-813b-7d858e6ec637" xlink:to="loc_nmih_OtherAssetBackedSecuritiesMember_019d951c-8312-742d-9f8a-69d889605e1c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermInvestmentsMember_019c540a-9e2a-72d4-9be2-7f52d07173ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermInvestmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-74a8-813b-7d858e6ec637" xlink:to="loc_us-gaap_ShortTermInvestmentsMember_019c540a-9e2a-72d4-9be2-7f52d07173ad" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions_019c540a-9e2a-7f62-b05c-9e2b71dd0bd1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7193-8737-3df769d9fe5e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions_019c540a-9e2a-7f62-b05c-9e2b71dd0bd1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019c540a-9e2a-7481-a68b-6022f3ae6fdc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7193-8737-3df769d9fe5e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019c540a-9e2a-7481-a68b-6022f3ae6fdc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019c540a-9e2a-7c68-92f3-b64d61b97d20" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7193-8737-3df769d9fe5e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019c540a-9e2a-7c68-92f3-b64d61b97d20" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions_019c540a-9e2a-768f-8aff-ce0351321823" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7193-8737-3df769d9fe5e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions_019c540a-9e2a-768f-8aff-ce0351321823" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_019c540a-9e2a-7e0f-956f-532c56b66198" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7193-8737-3df769d9fe5e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_019c540a-9e2a-7e0f-956f-532c56b66198" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019c540a-9e2a-709f-adbc-6ed90e1fa5dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7193-8737-3df769d9fe5e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019c540a-9e2a-709f-adbc-6ed90e1fa5dc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions_019c540a-9e2a-7f80-bc2f-3b49d2fffb94" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7193-8737-3df769d9fe5e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions_019c540a-9e2a-7f80-bc2f-3b49d2fffb94" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_019c540a-9e2a-717c-b10c-ac8660f2e928" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7193-8737-3df769d9fe5e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_019c540a-9e2a-717c-b10c-ac8660f2e928" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019c540a-9e2a-70c5-bdf6-27f8c6cc7a6e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c540a-9e2a-7193-8737-3df769d9fe5e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019c540a-9e2a-70c5-bdf6-27f8c6cc7a6e" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/InvestmentsScheduleofNetInvestmentIncomeDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#InvestmentsScheduleofNetInvestmentIncomeDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/InvestmentsScheduleofNetInvestmentIncomeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryLineItems_019c540a-9e2a-7f63-9308-04cdcc2b70d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryTable_019c540a-9e2a-7140-a0be-c200879b10a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryLineItems_019c540a-9e2a-7f63-9308-04cdcc2b70d6" xlink:to="loc_us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryTable_019c540a-9e2a-7140-a0be-c200879b10a9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_019c540a-9e2a-7c3b-9b3b-6085380ad00b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryTable_019c540a-9e2a-7140-a0be-c200879b10a9" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_019c540a-9e2a-7c3b-9b3b-6085380ad00b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c540a-9e2a-7c3b-9b3b-6085380ad00b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019c540a-9e2a-7c3b-9b3b-6085380ad00b" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c540a-9e2a-7c3b-9b3b-6085380ad00b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c540a-9e2a-7c56-a675-fb17fed286a3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019c540a-9e2a-7c3b-9b3b-6085380ad00b" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c540a-9e2a-7c56-a675-fb17fed286a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsMember_019c540a-9e2a-7b91-9eb7-8ab7cb8ebe58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c540a-9e2a-7c56-a675-fb17fed286a3" xlink:to="loc_us-gaap_CashAndCashEquivalentsMember_019c540a-9e2a-7b91-9eb7-8ab7cb8ebe58" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeNetAbstract_019c540a-9e2a-7581-8d5d-2bcdb97abf65" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfInvestmentIncomeReportedAmountsByCategoryLineItems_019c540a-9e2a-7f63-9308-04cdcc2b70d6" xlink:to="loc_us-gaap_InvestmentIncomeNetAbstract_019c540a-9e2a-7581-8d5d-2bcdb97abf65" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInterestAndDividend_019c540a-9e2a-776b-8c88-63508decfe55" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeInterestAndDividend"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentIncomeNetAbstract_019c540a-9e2a-7581-8d5d-2bcdb97abf65" xlink:to="loc_us-gaap_InvestmentIncomeInterestAndDividend_019c540a-9e2a-776b-8c88-63508decfe55" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInvestmentExpense_019c540a-9e2a-7ceb-8b3d-2f699948bab5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeInvestmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentIncomeNetAbstract_019c540a-9e2a-7581-8d5d-2bcdb97abf65" xlink:to="loc_us-gaap_InvestmentIncomeInvestmentExpense_019c540a-9e2a-7ceb-8b3d-2f699948bab5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeNet_019c540a-9e2a-78cd-82dd-39847b9200de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentIncomeNetAbstract_019c540a-9e2a-7581-8d5d-2bcdb97abf65" xlink:to="loc_us-gaap_InvestmentIncomeNet_019c540a-9e2a-78cd-82dd-39847b9200de" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/FairValueofFinancialInstrumentsScheduleofFairValueMeasurementsofFinancialInstrumentsDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#FairValueofFinancialInstrumentsScheduleofFairValueMeasurementsofFinancialInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/FairValueofFinancialInstrumentsScheduleofFairValueMeasurementsofFinancialInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c540a-9e2a-7713-b9ce-583bef32c5ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c540a-9e2a-7090-953c-9ec608e9eef2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c540a-9e2a-7713-b9ce-583bef32c5ae" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c540a-9e2a-7090-953c-9ec608e9eef2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019c540a-9e2a-7863-af42-8134cc5d91d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c540a-9e2a-7090-953c-9ec608e9eef2" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019c540a-9e2a-7863-af42-8134cc5d91d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019c540a-9e2a-7863-af42-8134cc5d91d4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019c540a-9e2a-7863-af42-8134cc5d91d4" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019c540a-9e2a-7863-af42-8134cc5d91d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019c540a-9e2a-7ab0-9f44-fd1c7fb5b819" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019c540a-9e2a-7863-af42-8134cc5d91d4" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019c540a-9e2a-7ab0-9f44-fd1c7fb5b819" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_019c540a-9e2a-70be-be3b-4c9d555ee292" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_019c540a-9e2a-7ab0-9f44-fd1c7fb5b819" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_019c540a-9e2a-70be-be3b-4c9d555ee292" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c540a-9e2a-71dd-aa5a-fa759b8d71ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c540a-9e2a-7090-953c-9ec608e9eef2" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c540a-9e2a-71dd-aa5a-fa759b8d71ec" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c540a-9e2a-71dd-aa5a-fa759b8d71ec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c540a-9e2a-71dd-aa5a-fa759b8d71ec" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c540a-9e2a-71dd-aa5a-fa759b8d71ec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c540a-9e2a-7238-8275-082e49a3169a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c540a-9e2a-71dd-aa5a-fa759b8d71ec" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c540a-9e2a-7238-8275-082e49a3169a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019c540a-9e2a-7a7f-801b-410a97085184" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c540a-9e2a-7238-8275-082e49a3169a" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019c540a-9e2a-7a7f-801b-410a97085184" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019c540a-9e2a-7dd8-9546-33f7089fbda3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c540a-9e2a-7238-8275-082e49a3169a" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019c540a-9e2a-7dd8-9546-33f7089fbda3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019c540a-9e2a-7842-ae94-ac2ba99fab21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c540a-9e2a-7238-8275-082e49a3169a" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019c540a-9e2a-7842-ae94-ac2ba99fab21" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-7dd8-98df-1949c5d8ab17" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c540a-9e2a-7090-953c-9ec608e9eef2" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-7dd8-98df-1949c5d8ab17" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7dd8-98df-1949c5d8ab17_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-7dd8-98df-1949c5d8ab17" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7dd8-98df-1949c5d8ab17_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7746-a5d7-08523c017e13" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c540a-9e2a-7dd8-98df-1949c5d8ab17" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7746-a5d7-08523c017e13" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_019c540a-9e2a-7c68-a0a5-c73d0ef02361" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7746-a5d7-08523c017e13" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_019c540a-9e2a-7c68-a0a5-c73d0ef02361" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019c540a-9e2a-7b45-bdbc-16cc667b88c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7746-a5d7-08523c017e13" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_019c540a-9e2a-7b45-bdbc-16cc667b88c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019c540a-9e2a-7105-ad71-9e0889175f9b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7746-a5d7-08523c017e13" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019c540a-9e2a-7105-ad71-9e0889175f9b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cf6d6-584d-79c2-a59f-fc8ed8e88da4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7746-a5d7-08523c017e13" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019cf6d6-584d-79c2-a59f-fc8ed8e88da4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_OtherAssetBackedSecuritiesMember_019d95cc-a738-7da1-93ba-37ddfea2c7eb" xlink:href="nmih-20260331.xsd#nmih_OtherAssetBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c540a-9e2a-7746-a5d7-08523c017e13" xlink:to="loc_nmih_OtherAssetBackedSecuritiesMember_019d95cc-a738-7da1-93ba-37ddfea2c7eb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019c540a-9e2a-7eab-9c81-3d851f3f18fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c540a-9e2a-7713-b9ce-583bef32c5ae" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019c540a-9e2a-7eab-9c81-3d851f3f18fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019c540a-9e2a-7316-9263-02403105994a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c540a-9e2a-7713-b9ce-583bef32c5ae" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019c540a-9e2a-7316-9263-02403105994a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019c540a-9e2a-7df9-946b-8f525e5ef0a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c540a-9e2a-7713-b9ce-583bef32c5ae" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019c540a-9e2a-7df9-946b-8f525e5ef0a0" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/FairValueofFinancialInstrumentsNarrativeDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#FairValueofFinancialInstrumentsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/FairValueofFinancialInstrumentsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c540a-9e2a-7278-aa48-82e5b70cdde7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c540a-9e2a-798c-9a3b-c8a1b72bf49c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c540a-9e2a-7278-aa48-82e5b70cdde7" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c540a-9e2a-798c-9a3b-c8a1b72bf49c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019c540a-9e2a-70e3-8b23-773888838ec2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c540a-9e2a-798c-9a3b-c8a1b72bf49c" xlink:to="loc_us-gaap_DebtInstrumentAxis_019c540a-9e2a-70e3-8b23-773888838ec2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-70e3-8b23-773888838ec2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019c540a-9e2a-70e3-8b23-773888838ec2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-70e3-8b23-773888838ec2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-7fa0-b3bd-eb740d41409d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019c540a-9e2a-70e3-8b23-773888838ec2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-7fa0-b3bd-eb740d41409d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_Senior2024NotesMember_019c540a-9e2a-7b09-bd66-3f25dd96a9d2" xlink:href="nmih-20260331.xsd#nmih_Senior2024NotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-7fa0-b3bd-eb740d41409d" xlink:to="loc_nmih_Senior2024NotesMember_019c540a-9e2a-7b09-bd66-3f25dd96a9d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019c540a-9e2a-7e23-86ba-276f42dcf6f1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c540a-9e2a-798c-9a3b-c8a1b72bf49c" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019c540a-9e2a-7e23-86ba-276f42dcf6f1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019c540a-9e2a-7e23-86ba-276f42dcf6f1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019c540a-9e2a-7e23-86ba-276f42dcf6f1" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019c540a-9e2a-7e23-86ba-276f42dcf6f1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019c540a-9e2a-7352-9897-b4bde35f36ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019c540a-9e2a-7e23-86ba-276f42dcf6f1" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019c540a-9e2a-7352-9897-b4bde35f36ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnsecuredDebtMember_019c540a-9e2a-72ff-9c15-100765808e69" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnsecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019c540a-9e2a-7352-9897-b4bde35f36ac" xlink:to="loc_us-gaap_UnsecuredDebtMember_019c540a-9e2a-72ff-9c15-100765808e69" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019c540a-9e2a-7b11-9b41-0a55f1b8bc52" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019c540a-9e2a-7352-9897-b4bde35f36ac" xlink:to="loc_us-gaap_SecuredDebtMember_019c540a-9e2a-7b11-9b41-0a55f1b8bc52" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c540a-9e2a-72c0-955f-a4a1752c9bee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c540a-9e2a-798c-9a3b-c8a1b72bf49c" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c540a-9e2a-72c0-955f-a4a1752c9bee" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c540a-9e2a-72c0-955f-a4a1752c9bee_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c540a-9e2a-72c0-955f-a4a1752c9bee" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c540a-9e2a-72c0-955f-a4a1752c9bee_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c540a-9e2a-7cc4-99c6-310dae419a49" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c540a-9e2a-72c0-955f-a4a1752c9bee" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c540a-9e2a-7cc4-99c6-310dae419a49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019c540a-9e2a-768a-9c42-62544645bba8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c540a-9e2a-7cc4-99c6-310dae419a49" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019c540a-9e2a-768a-9c42-62544645bba8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019c540a-9e2a-7638-84d1-4e7bf65f1991" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c540a-9e2a-7278-aa48-82e5b70cdde7" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019c540a-9e2a-7638-84d1-4e7bf65f1991" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019c540a-9e2a-719c-9ed5-f00ea0e23fb1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c540a-9e2a-7278-aa48-82e5b70cdde7" xlink:to="loc_us-gaap_LongTermDebt_019c540a-9e2a-719c-9ed5-f00ea0e23fb1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_019c540a-9e2a-720a-97a9-5f4fcdf0cf24" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c540a-9e2a-7278-aa48-82e5b70cdde7" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_019c540a-9e2a-720a-97a9-5f4fcdf0cf24" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_019c540a-9e2a-71cc-87d8-8db3b93481ed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c540a-9e2a-7278-aa48-82e5b70cdde7" xlink:to="loc_us-gaap_LongTermDebtFairValue_019c540a-9e2a-71cc-87d8-8db3b93481ed" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/DebtDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#DebtDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/DebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019c540a-9e2a-7a47-a1e3-7de3dfa7767e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_DebtInstrumentTable_019c540a-9e2a-7a47-a1e3-7de3dfa7767e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019c540a-9e2a-736a-bc82-766a6de07cd4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c540a-9e2a-7a47-a1e3-7de3dfa7767e" xlink:to="loc_us-gaap_DebtInstrumentAxis_019c540a-9e2a-736a-bc82-766a6de07cd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-736a-bc82-766a6de07cd4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019c540a-9e2a-736a-bc82-766a6de07cd4" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-736a-bc82-766a6de07cd4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-76ef-a4bc-b3397d304653" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019c540a-9e2a-736a-bc82-766a6de07cd4" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-76ef-a4bc-b3397d304653" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_Senior2024NotesMember_019c540a-9e2a-7a35-9a8f-84efe03877dc" xlink:href="nmih-20260331.xsd#nmih_Senior2024NotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-76ef-a4bc-b3397d304653" xlink:to="loc_nmih_Senior2024NotesMember_019c540a-9e2a-7a35-9a8f-84efe03877dc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_A2024RevolvingCreditFacilityMember_019c540a-9e2a-7916-a881-2c5935332468" xlink:href="nmih-20260331.xsd#nmih_A2024RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-76ef-a4bc-b3397d304653" xlink:to="loc_nmih_A2024RevolvingCreditFacilityMember_019c540a-9e2a-7916-a881-2c5935332468" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_SecuredRevolvingCreditFacility2021Member_019c540a-9e2a-7338-878b-f002114a5fd4" xlink:href="nmih-20260331.xsd#nmih_SecuredRevolvingCreditFacility2021Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-76ef-a4bc-b3397d304653" xlink:to="loc_nmih_SecuredRevolvingCreditFacility2021Member_019c540a-9e2a-7338-878b-f002114a5fd4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_SecuredRevolvingCreditFacility2024Member_019cf6ec-0c0a-7b86-8718-b6c0e4bee440" xlink:href="nmih-20260331.xsd#nmih_SecuredRevolvingCreditFacility2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-76ef-a4bc-b3397d304653" xlink:to="loc_nmih_SecuredRevolvingCreditFacility2024Member_019cf6ec-0c0a-7b86-8718-b6c0e4bee440" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_RevolvingCreditFacility2024Member_019cf6ec-0c0a-7e8a-8ef1-5b649adb5631" xlink:href="nmih-20260331.xsd#nmih_RevolvingCreditFacility2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c540a-9e2a-76ef-a4bc-b3397d304653" xlink:to="loc_nmih_RevolvingCreditFacility2024Member_019cf6ec-0c0a-7e8a-8ef1-5b649adb5631" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019c540a-9e2a-7df9-9178-aee6680a885b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c540a-9e2a-7a47-a1e3-7de3dfa7767e" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019c540a-9e2a-7df9-9178-aee6680a885b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019c540a-9e2a-7df9-9178-aee6680a885b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019c540a-9e2a-7df9-9178-aee6680a885b" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019c540a-9e2a-7df9-9178-aee6680a885b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019c540a-9e2a-7ff7-a9a9-9f3037fff769" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019c540a-9e2a-7df9-9178-aee6680a885b" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019c540a-9e2a-7ff7-a9a9-9f3037fff769" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnsecuredDebtMember_019c540a-9e2a-7d9a-8530-a52a8b84d21d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnsecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019c540a-9e2a-7ff7-a9a9-9f3037fff769" xlink:to="loc_us-gaap_UnsecuredDebtMember_019c540a-9e2a-7d9a-8530-a52a8b84d21d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019c540a-9e2a-7565-b4bc-706b4326de46" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019c540a-9e2a-7ff7-a9a9-9f3037fff769" xlink:to="loc_us-gaap_SecuredDebtMember_019c540a-9e2a-7565-b4bc-706b4326de46" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_019c540a-9e2a-7117-a417-f692a1d45c38" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c540a-9e2a-7a47-a1e3-7de3dfa7767e" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_019c540a-9e2a-7117-a417-f692a1d45c38" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_019c540a-9e2a-7117-a417-f692a1d45c38_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_019c540a-9e2a-7117-a417-f692a1d45c38" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_019c540a-9e2a-7117-a417-f692a1d45c38_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_019c540a-9e2a-7d18-9a2e-c161f30f827e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_019c540a-9e2a-7117-a417-f692a1d45c38" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_019c540a-9e2a-7d18-9a2e-c161f30f827e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_OnOrAfterJuly152029Member_019c540a-9e2a-77df-b538-fa5330b6e9ef" xlink:href="nmih-20260331.xsd#nmih_OnOrAfterJuly152029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_019c540a-9e2a-7d18-9a2e-c161f30f827e" xlink:to="loc_nmih_OnOrAfterJuly152029Member_019c540a-9e2a-77df-b538-fa5330b6e9ef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019c540a-9e2a-7804-bac2-c64f65752eaf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c540a-9e2a-7a47-a1e3-7de3dfa7767e" xlink:to="loc_us-gaap_CreditFacilityAxis_019c540a-9e2a-7804-bac2-c64f65752eaf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019c540a-9e2a-7804-bac2-c64f65752eaf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019c540a-9e2a-7804-bac2-c64f65752eaf" xlink:to="loc_us-gaap_CreditFacilityDomain_019c540a-9e2a-7804-bac2-c64f65752eaf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019c540a-9e2a-72d7-a4e0-59b4ab01dd32" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019c540a-9e2a-7804-bac2-c64f65752eaf" xlink:to="loc_us-gaap_CreditFacilityDomain_019c540a-9e2a-72d7-a4e0-59b4ab01dd32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019c540a-9e2a-74dc-95d7-ca5b3a953938" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019c540a-9e2a-72d7-a4e0-59b4ab01dd32" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019c540a-9e2a-74dc-95d7-ca5b3a953938" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_019c540a-9e2a-7fcd-a6eb-a470871fed79" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c540a-9e2a-7a47-a1e3-7de3dfa7767e" xlink:to="loc_us-gaap_VariableRateAxis_019c540a-9e2a-7fcd-a6eb-a470871fed79" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019c540a-9e2a-7fcd-a6eb-a470871fed79_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_019c540a-9e2a-7fcd-a6eb-a470871fed79" xlink:to="loc_us-gaap_VariableRateDomain_019c540a-9e2a-7fcd-a6eb-a470871fed79_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019c540a-9e2a-7aae-aba8-3e8d06d8cced" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_019c540a-9e2a-7fcd-a6eb-a470871fed79" xlink:to="loc_us-gaap_VariableRateDomain_019c540a-9e2a-7aae-aba8-3e8d06d8cced" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_019c540a-9e2a-7215-a13b-3cb542f0bf20" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019c540a-9e2a-7aae-aba8-3e8d06d8cced" xlink:to="loc_us-gaap_BaseRateMember_019c540a-9e2a-7215-a13b-3cb542f0bf20" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019c540a-9e2a-73a6-91b0-e74bdd708894" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019c540a-9e2a-7aae-aba8-3e8d06d8cced" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019c540a-9e2a-73a6-91b0-e74bdd708894" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019c540a-9e2a-70bb-aa1a-dd81c9072c85" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c540a-9e2a-7a47-a1e3-7de3dfa7767e" xlink:to="loc_srt_RangeAxis_019c540a-9e2a-70bb-aa1a-dd81c9072c85" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c540a-9e2a-70bb-aa1a-dd81c9072c85_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019c540a-9e2a-70bb-aa1a-dd81c9072c85" xlink:to="loc_srt_RangeMember_019c540a-9e2a-70bb-aa1a-dd81c9072c85_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c540a-9e2a-7213-8bd3-3a7bc62841a6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019c540a-9e2a-70bb-aa1a-dd81c9072c85" xlink:to="loc_srt_RangeMember_019c540a-9e2a-7213-8bd3-3a7bc62841a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019c540a-9e2a-79b5-aed0-2b45a7795659" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c540a-9e2a-7213-8bd3-3a7bc62841a6" xlink:to="loc_srt_MinimumMember_019c540a-9e2a-79b5-aed0-2b45a7795659" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019c540a-9e2a-76aa-a67c-dddbecaf2721" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c540a-9e2a-7213-8bd3-3a7bc62841a6" xlink:to="loc_srt_MaximumMember_019c540a-9e2a-76aa-a67c-dddbecaf2721" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotes_019c540a-9e2a-7027-8e2e-bf7c3ab486e6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeniorNotes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_SeniorNotes_019c540a-9e2a-7027-8e2e-bf7c3ab486e6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019c540a-9e2a-75bc-a2f1-3724c6d658cd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019c540a-9e2a-75bc-a2f1-3724c6d658cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_019c540a-9e2a-76ae-bf08-38831d516d26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPricePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_019c540a-9e2a-76ae-bf08-38831d516d26" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_019c540a-9e2a-726b-b765-c6daab268a7c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_019c540a-9e2a-726b-b765-c6daab268a7c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019c540a-9e2a-7101-b146-a467fa6a16bb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019c540a-9e2a-7101-b146-a467fa6a16bb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPayableCurrentAndNoncurrent_019c540a-9e2a-7351-aca5-2fbc98bb2759" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPayableCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_InterestPayableCurrentAndNoncurrent_019c540a-9e2a-7351-aca5-2fbc98bb2759" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019c540a-9e2a-7898-a0be-0b2d22bb4d6c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019c540a-9e2a-7898-a0be-0b2d22bb4d6c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_019c540a-9e2a-7097-baa0-8c769b8c4536" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_DebtInstrumentTerm_019c540a-9e2a-7097-baa0-8c769b8c4536" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_DebtInstrumentVariableInterestRateFloor_019c540a-9e2a-7021-8da8-4d84e674bbb2" xlink:href="nmih-20260331.xsd#nmih_DebtInstrumentVariableInterestRateFloor"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_nmih_DebtInstrumentVariableInterestRateFloor_019c540a-9e2a-7021-8da8-4d84e674bbb2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019c540a-9e2a-7de3-9280-eaa7befc01a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019c540a-9e2a-7de3-9280-eaa7befc01a0" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_019c540a-9e2a-7ead-9edc-14612b3299a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_LineOfCredit_019c540a-9e2a-7ead-9edc-14612b3299a1" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019c540a-9e2a-77b0-a44b-2a3a9a343fee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019c540a-9e2a-77b0-a44b-2a3a9a343fee" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityCommitmentFeeAmount_019c540a-9e2a-70d6-b30d-180a8917455c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityCommitmentFeeAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_LineOfCreditFacilityCommitmentFeeAmount_019c540a-9e2a-70d6-b30d-180a8917455c" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtIssuanceCostsLineOfCreditArrangementsGross_019c540a-9e2a-794f-8846-31b16dc4d3c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtIssuanceCostsLineOfCreditArrangementsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_DebtIssuanceCostsLineOfCreditArrangementsGross_019c540a-9e2a-794f-8846-31b16dc4d3c7" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsGross_019cf6e9-9455-72de-bd1a-884435d3d9ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_DeferredFinanceCostsGross_019cf6e9-9455-72de-bd1a-884435d3d9ec" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet_019c540a-9e2a-7e82-98a8-bb5b32271fc0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet_019c540a-9e2a-7e82-98a8-bb5b32271fc0" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_DebtInstrumentCovenantTermsDebtToTotalCapitalizationRatioMaximum_019c540a-9e2a-70ce-84a1-cf3aa89f6783" xlink:href="nmih-20260331.xsd#nmih_DebtInstrumentCovenantTermsDebtToTotalCapitalizationRatioMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c540a-9e2a-7643-9c59-44e1ff9102db" xlink:to="loc_nmih_DebtInstrumentCovenantTermsDebtToTotalCapitalizationRatioMaximum_019c540a-9e2a-70ce-84a1-cf3aa89f6783" xlink:type="arc" order="16"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/ReinsuranceNarrativeDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#ReinsuranceNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/ReinsuranceNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7472-a64e-9cfa9c230f73" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7472-a64e-9cfa9c230f73" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-745f-bf4e-b629cfd70e67" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7472-a64e-9cfa9c230f73" xlink:to="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-745f-bf4e-b629cfd70e67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-745f-bf4e-b629cfd70e67_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-745f-bf4e-b629cfd70e67" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-745f-bf4e-b629cfd70e67_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-74fc-98e5-5a409cf6d0bc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-745f-bf4e-b629cfd70e67" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-74fc-98e5-5a409cf6d0bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ThirdPartyReinsurersMember_019c540a-9e2a-7b89-a62f-323e8cd91ab5" xlink:href="nmih-20260331.xsd#nmih_ThirdPartyReinsurersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-74fc-98e5-5a409cf6d0bc" xlink:to="loc_nmih_ThirdPartyReinsurersMember_019c540a-9e2a-7b89-a62f-323e8cd91ab5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsuranceContractAxis_019c540a-9e2a-7bdc-925c-899701f16d03" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReinsuranceContractAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7472-a64e-9cfa9c230f73" xlink:to="loc_us-gaap_ReinsuranceContractAxis_019c540a-9e2a-7bdc-925c-899701f16d03" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019c540a-9e2a-704e-b261-25f74674b9d6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7472-a64e-9cfa9c230f73" xlink:to="loc_srt_RangeAxis_019c540a-9e2a-704e-b261-25f74674b9d6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c540a-9e2a-704e-b261-25f74674b9d6_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019c540a-9e2a-704e-b261-25f74674b9d6" xlink:to="loc_srt_RangeMember_019c540a-9e2a-704e-b261-25f74674b9d6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c540a-9e2a-77f8-a0a3-e21295c375ea" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019c540a-9e2a-704e-b261-25f74674b9d6" xlink:to="loc_srt_RangeMember_019c540a-9e2a-77f8-a0a3-e21295c375ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019c540a-9e2a-726d-ba2d-a25c832491db" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c540a-9e2a-77f8-a0a3-e21295c375ea" xlink:to="loc_srt_MinimumMember_019c540a-9e2a-726d-ba2d-a25c832491db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019c540a-9e2a-7dbf-855a-4f31675ee61d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c540a-9e2a-77f8-a0a3-e21295c375ea" xlink:to="loc_srt_MaximumMember_019c540a-9e2a-7dbf-855a-4f31675ee61d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019c540a-9e2a-7972-aa56-bcba0db8fb1e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7472-a64e-9cfa9c230f73" xlink:to="loc_srt_CounterpartyNameAxis_019c540a-9e2a-7972-aa56-bcba0db8fb1e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c540a-9e2a-7972-aa56-bcba0db8fb1e_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019c540a-9e2a-7972-aa56-bcba0db8fb1e" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c540a-9e2a-7972-aa56-bcba0db8fb1e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c540a-9e2a-7357-b280-9cc853456524" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019c540a-9e2a-7972-aa56-bcba0db8fb1e" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c540a-9e2a-7357-b280-9cc853456524" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_A20262XOLTransactionMember_019c540a-9e2a-7ce6-b04e-461778f17a5c" xlink:href="nmih-20260331.xsd#nmih_A20262XOLTransactionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c540a-9e2a-7357-b280-9cc853456524" xlink:to="loc_nmih_A20262XOLTransactionMember_019c540a-9e2a-7ce6-b04e-461778f17a5c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_A2027XOLTransactionMember_019cf756-beb0-70bd-acb3-b85d59564bd9" xlink:href="nmih-20260331.xsd#nmih_A2027XOLTransactionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c540a-9e2a-7357-b280-9cc853456524" xlink:to="loc_nmih_A2027XOLTransactionMember_019cf756-beb0-70bd-acb3-b85d59564bd9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_NumberOfQuotaShareReinsuranceTreaties_019c540a-9e2a-7753-be1a-9e85180ee823" xlink:href="nmih-20260331.xsd#nmih_NumberOfQuotaShareReinsuranceTreaties"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_NumberOfQuotaShareReinsuranceTreaties_019c540a-9e2a-7753-be1a-9e85180ee823" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_PremiumsEarnedPercentOfRisk_019c540a-9e2a-77bb-9a12-c12b28a5ec56" xlink:href="nmih-20260331.xsd#nmih_PremiumsEarnedPercentOfRisk"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_PremiumsEarnedPercentOfRisk_019c540a-9e2a-77bb-9a12-c12b28a5ec56" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses_019c540a-9e2a-7fda-aac1-4a0c0fdcc3d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses_019c540a-9e2a-7fda-aac1-4a0c0fdcc3d0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_NumberOfExcessOfLossReinsuranceAgreements_019c540a-9e2a-7d57-88ab-3983a209fffa" xlink:href="nmih-20260331.xsd#nmih_NumberOfExcessOfLossReinsuranceAgreements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_NumberOfExcessOfLossReinsuranceAgreements_019c540a-9e2a-7d57-88ab-3983a209fffa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceRetentionPolicyTermOfUnderlyingMortgageAmortization_019c540a-9e2a-766c-b47e-4fd9a5d6eb53" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceRetentionPolicyTermOfUnderlyingMortgageAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_ReinsuranceRetentionPolicyTermOfUnderlyingMortgageAmortization_019c540a-9e2a-766c-b47e-4fd9a5d6eb53" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceRetentionPolicyCededAggregatePremiums_019c540a-9e2a-7e29-a5a4-349b5d8b2502" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceRetentionPolicyCededAggregatePremiums"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_ReinsuranceRetentionPolicyCededAggregatePremiums_019c540a-9e2a-7e29-a5a4-349b5d8b2502" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceRetentionPolicyExcessOfLossReinsuranceCoverages_019c540a-9e2a-7c8f-8749-3bbff1d3cf11" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceRetentionPolicyExcessOfLossReinsuranceCoverages"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_ReinsuranceRetentionPolicyExcessOfLossReinsuranceCoverages_019c540a-9e2a-7c8f-8749-3bbff1d3cf11" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_AnticipatedOperatingExpensePaymentsAnnual_019c540a-9e2a-7b33-afd2-1444b3a5ae31" xlink:href="nmih-20260331.xsd#nmih_AnticipatedOperatingExpensePaymentsAnnual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_AnticipatedOperatingExpensePaymentsAnnual_019c540a-9e2a-7b33-afd2-1444b3a5ae31" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceRetentionRiskPremiumsPaid_019c540a-9e2a-77c8-9694-c5436e5ad249" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceRetentionRiskPremiumsPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_ReinsuranceRetentionRiskPremiumsPaid_019c540a-9e2a-77c8-9694-c5436e5ad249" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceAgreementOptionalTerminationRightPercentOfReinsuranceCoverageThreshold_019c540a-9e2a-7ef3-b7fe-9bf29981e978" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceAgreementOptionalTerminationRightPercentOfReinsuranceCoverageThreshold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_ReinsuranceAgreementOptionalTerminationRightPercentOfReinsuranceCoverageThreshold_019c540a-9e2a-7ef3-b7fe-9bf29981e978" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/ReinsuranceNarrativeDetails_1" xlink:type="simple" xlink:href="nmih-20260331.xsd#ReinsuranceNarrativeDetails_1"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/ReinsuranceNarrativeDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7472-a64e-9cfa9c230f73" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7472-a64e-9cfa9c230f73" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-745f-bf4e-b629cfd70e67" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7472-a64e-9cfa9c230f73" xlink:to="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-745f-bf4e-b629cfd70e67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-745f-bf4e-b629cfd70e67_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-745f-bf4e-b629cfd70e67" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-745f-bf4e-b629cfd70e67_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-74fc-98e5-5a409cf6d0bc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-745f-bf4e-b629cfd70e67" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-74fc-98e5-5a409cf6d0bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ThirdPartyReinsurersMember_019c540a-9e2a-7b89-a62f-323e8cd91ab5" xlink:href="nmih-20260331.xsd#nmih_ThirdPartyReinsurersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-74fc-98e5-5a409cf6d0bc" xlink:to="loc_nmih_ThirdPartyReinsurersMember_019c540a-9e2a-7b89-a62f-323e8cd91ab5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019c540a-9e2a-704e-b261-25f74674b9d6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7472-a64e-9cfa9c230f73" xlink:to="loc_srt_RangeAxis_019c540a-9e2a-704e-b261-25f74674b9d6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c540a-9e2a-704e-b261-25f74674b9d6_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019c540a-9e2a-704e-b261-25f74674b9d6" xlink:to="loc_srt_RangeMember_019c540a-9e2a-704e-b261-25f74674b9d6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c540a-9e2a-77f8-a0a3-e21295c375ea" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019c540a-9e2a-704e-b261-25f74674b9d6" xlink:to="loc_srt_RangeMember_019c540a-9e2a-77f8-a0a3-e21295c375ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019c540a-9e2a-726d-ba2d-a25c832491db" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c540a-9e2a-77f8-a0a3-e21295c375ea" xlink:to="loc_srt_MinimumMember_019c540a-9e2a-726d-ba2d-a25c832491db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019c540a-9e2a-7dbf-855a-4f31675ee61d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c540a-9e2a-77f8-a0a3-e21295c375ea" xlink:to="loc_srt_MaximumMember_019c540a-9e2a-7dbf-855a-4f31675ee61d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019c540a-9e2a-7972-aa56-bcba0db8fb1e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7472-a64e-9cfa9c230f73" xlink:to="loc_srt_CounterpartyNameAxis_019c540a-9e2a-7972-aa56-bcba0db8fb1e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c540a-9e2a-7972-aa56-bcba0db8fb1e_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019c540a-9e2a-7972-aa56-bcba0db8fb1e" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c540a-9e2a-7972-aa56-bcba0db8fb1e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c540a-9e2a-7357-b280-9cc853456524" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019c540a-9e2a-7972-aa56-bcba0db8fb1e" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c540a-9e2a-7357-b280-9cc853456524" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_A20262XOLTransactionMember_019c540a-9e2a-7ce6-b04e-461778f17a5c" xlink:href="nmih-20260331.xsd#nmih_A20262XOLTransactionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c540a-9e2a-7357-b280-9cc853456524" xlink:to="loc_nmih_A20262XOLTransactionMember_019c540a-9e2a-7ce6-b04e-461778f17a5c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_A2027XOLTransactionMember_019cf756-beb0-70bd-acb3-b85d59564bd9" xlink:href="nmih-20260331.xsd#nmih_A2027XOLTransactionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c540a-9e2a-7357-b280-9cc853456524" xlink:to="loc_nmih_A2027XOLTransactionMember_019cf756-beb0-70bd-acb3-b85d59564bd9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_NumberOfQuotaShareReinsuranceTreaties_019c540a-9e2a-7753-be1a-9e85180ee823" xlink:href="nmih-20260331.xsd#nmih_NumberOfQuotaShareReinsuranceTreaties"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_NumberOfQuotaShareReinsuranceTreaties_019c540a-9e2a-7753-be1a-9e85180ee823" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_PremiumsEarnedPercentOfRisk_019c540a-9e2a-77bb-9a12-c12b28a5ec56" xlink:href="nmih-20260331.xsd#nmih_PremiumsEarnedPercentOfRisk"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_PremiumsEarnedPercentOfRisk_019c540a-9e2a-77bb-9a12-c12b28a5ec56" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses_019c540a-9e2a-7fda-aac1-4a0c0fdcc3d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_us-gaap_ReinsuranceRecoverablesOnPaidAndUnpaidLosses_019c540a-9e2a-7fda-aac1-4a0c0fdcc3d0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_NumberOfExcessOfLossReinsuranceAgreements_019c540a-9e2a-7d57-88ab-3983a209fffa" xlink:href="nmih-20260331.xsd#nmih_NumberOfExcessOfLossReinsuranceAgreements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_NumberOfExcessOfLossReinsuranceAgreements_019c540a-9e2a-7d57-88ab-3983a209fffa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceRetentionPolicyTermOfUnderlyingMortgageAmortization_019c540a-9e2a-766c-b47e-4fd9a5d6eb53" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceRetentionPolicyTermOfUnderlyingMortgageAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_ReinsuranceRetentionPolicyTermOfUnderlyingMortgageAmortization_019c540a-9e2a-766c-b47e-4fd9a5d6eb53" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceRetentionPolicyCededAggregatePremiums_019c540a-9e2a-7e29-a5a4-349b5d8b2502" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceRetentionPolicyCededAggregatePremiums"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_ReinsuranceRetentionPolicyCededAggregatePremiums_019c540a-9e2a-7e29-a5a4-349b5d8b2502" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceRetentionPolicyExcessOfLossReinsuranceCoverages_019c540a-9e2a-7c8f-8749-3bbff1d3cf11" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceRetentionPolicyExcessOfLossReinsuranceCoverages"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_ReinsuranceRetentionPolicyExcessOfLossReinsuranceCoverages_019c540a-9e2a-7c8f-8749-3bbff1d3cf11" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_AnticipatedOperatingExpensePaymentsAnnual_019c540a-9e2a-7b33-afd2-1444b3a5ae31" xlink:href="nmih-20260331.xsd#nmih_AnticipatedOperatingExpensePaymentsAnnual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_AnticipatedOperatingExpensePaymentsAnnual_019c540a-9e2a-7b33-afd2-1444b3a5ae31" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceRetentionRiskPremiumsPaid_019c540a-9e2a-77c8-9694-c5436e5ad249" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceRetentionRiskPremiumsPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_ReinsuranceRetentionRiskPremiumsPaid_019c540a-9e2a-77c8-9694-c5436e5ad249" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceAgreementOptionalTerminationRightPercentOfReinsuranceCoverageThreshold_019c540a-9e2a-7ef3-b7fe-9bf29981e978" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceAgreementOptionalTerminationRightPercentOfReinsuranceCoverageThreshold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-77bf-bed0-0e5a7282d8a9" xlink:to="loc_nmih_ReinsuranceAgreementOptionalTerminationRightPercentOfReinsuranceCoverageThreshold_019c540a-9e2a-7ef3-b7fe-9bf29981e978" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/ReinsuranceScheduleoftheInceptionDateCoveredProductionPeriodContractualandOptionalTerminationDatesCurrentCessionRateCedingCommissionandProfitCommissionforEachOutstandingQSRTransactionDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#ReinsuranceScheduleoftheInceptionDateCoveredProductionPeriodContractualandOptionalTerminationDatesCurrentCessionRateCedingCommissionandProfitCommissionforEachOutstandingQSRTransactionDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/ReinsuranceScheduleoftheInceptionDateCoveredProductionPeriodContractualandOptionalTerminationDatesCurrentCessionRateCedingCommissionandProfitCommissionforEachOutstandingQSRTransactionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7562-a40d-4be14367bb00" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7f5e-bb46-834132424b97" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7562-a40d-4be14367bb00" xlink:to="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7f5e-bb46-834132424b97" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7f68-9f00-492bcf4945ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7f5e-bb46-834132424b97" xlink:to="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7f68-9f00-492bcf4945ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7f68-9f00-492bcf4945ae_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7f68-9f00-492bcf4945ae" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7f68-9f00-492bcf4945ae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7835-9c4c-2245e22e2d4c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7f68-9f00-492bcf4945ae" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7835-9c4c-2245e22e2d4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ThirdPartyReinsurersMember_019c540a-9e2a-703a-ad70-5c8b8cc8fcbd" xlink:href="nmih-20260331.xsd#nmih_ThirdPartyReinsurersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7835-9c4c-2245e22e2d4c" xlink:to="loc_nmih_ThirdPartyReinsurersMember_019c540a-9e2a-703a-ad70-5c8b8cc8fcbd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsuranceContractAxis_019c540a-9e2a-7fc3-a544-498ef98035a2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReinsuranceContractAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7f5e-bb46-834132424b97" xlink:to="loc_us-gaap_ReinsuranceContractAxis_019c540a-9e2a-7fc3-a544-498ef98035a2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_PremiumsEarnedPercentOfRisk_019c540a-9e2a-702c-a757-b40597c80d7b" xlink:href="nmih-20260331.xsd#nmih_PremiumsEarnedPercentOfRisk"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7562-a40d-4be14367bb00" xlink:to="loc_nmih_PremiumsEarnedPercentOfRisk_019c540a-9e2a-702c-a757-b40597c80d7b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_CededCreditRiskCommissionEarnedPercentage_019c540a-9e2a-717f-957a-2e0c300aaa2a" xlink:href="nmih-20260331.xsd#nmih_CededCreditRiskCommissionEarnedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7562-a40d-4be14367bb00" xlink:to="loc_nmih_CededCreditRiskCommissionEarnedPercentage_019c540a-9e2a-717f-957a-2e0c300aaa2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_CededCreditRiskProfitCommissionEarnedPercentage_019c540a-9e2a-7d14-816d-6702b1b41a9b" xlink:href="nmih-20260331.xsd#nmih_CededCreditRiskProfitCommissionEarnedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7562-a40d-4be14367bb00" xlink:to="loc_nmih_CededCreditRiskProfitCommissionEarnedPercentage_019c540a-9e2a-7d14-816d-6702b1b41a9b" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/ReinsuranceScheduleoftheInceptionDateCoveredProductionPeriodContractualandOptionalTerminationDatesCurrentCessionRateCedingCommissionandProfitCommissionforEachOutstandingQSRTransactionDetails_1" xlink:type="simple" xlink:href="nmih-20260331.xsd#ReinsuranceScheduleoftheInceptionDateCoveredProductionPeriodContractualandOptionalTerminationDatesCurrentCessionRateCedingCommissionandProfitCommissionforEachOutstandingQSRTransactionDetails_1"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/ReinsuranceScheduleoftheInceptionDateCoveredProductionPeriodContractualandOptionalTerminationDatesCurrentCessionRateCedingCommissionandProfitCommissionforEachOutstandingQSRTransactionDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7562-a40d-4be14367bb00" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7f5e-bb46-834132424b97" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7562-a40d-4be14367bb00" xlink:to="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7f5e-bb46-834132424b97" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7f68-9f00-492bcf4945ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-7f5e-bb46-834132424b97" xlink:to="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7f68-9f00-492bcf4945ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7f68-9f00-492bcf4945ae_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7f68-9f00-492bcf4945ae" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7f68-9f00-492bcf4945ae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7835-9c4c-2245e22e2d4c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7f68-9f00-492bcf4945ae" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7835-9c4c-2245e22e2d4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ThirdPartyReinsurersMember_019c540a-9e2a-703a-ad70-5c8b8cc8fcbd" xlink:href="nmih-20260331.xsd#nmih_ThirdPartyReinsurersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7835-9c4c-2245e22e2d4c" xlink:to="loc_nmih_ThirdPartyReinsurersMember_019c540a-9e2a-703a-ad70-5c8b8cc8fcbd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_PremiumsEarnedPercentOfRisk_019c540a-9e2a-702c-a757-b40597c80d7b" xlink:href="nmih-20260331.xsd#nmih_PremiumsEarnedPercentOfRisk"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7562-a40d-4be14367bb00" xlink:to="loc_nmih_PremiumsEarnedPercentOfRisk_019c540a-9e2a-702c-a757-b40597c80d7b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_CededCreditRiskCommissionEarnedPercentage_019c540a-9e2a-717f-957a-2e0c300aaa2a" xlink:href="nmih-20260331.xsd#nmih_CededCreditRiskCommissionEarnedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7562-a40d-4be14367bb00" xlink:to="loc_nmih_CededCreditRiskCommissionEarnedPercentage_019c540a-9e2a-717f-957a-2e0c300aaa2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_CededCreditRiskProfitCommissionEarnedPercentage_019c540a-9e2a-7d14-816d-6702b1b41a9b" xlink:href="nmih-20260331.xsd#nmih_CededCreditRiskProfitCommissionEarnedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7562-a40d-4be14367bb00" xlink:to="loc_nmih_CededCreditRiskProfitCommissionEarnedPercentage_019c540a-9e2a-7d14-816d-6702b1b41a9b" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/ReinsuranceScheduleofAmountsRelatedtoQSRTransactionsDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#ReinsuranceScheduleofAmountsRelatedtoQSRTransactionsDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/ReinsuranceScheduleofAmountsRelatedtoQSRTransactionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-76f0-be40-3c5c296dbe94" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:to="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-76f0-be40-3c5c296dbe94" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7cdb-ada8-4e26c4d8f930" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-76f0-be40-3c5c296dbe94" xlink:to="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7cdb-ada8-4e26c4d8f930" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7cdb-ada8-4e26c4d8f930_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7cdb-ada8-4e26c4d8f930" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7cdb-ada8-4e26c4d8f930_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7cfe-9b88-85f3c606be5e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7cdb-ada8-4e26c4d8f930" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7cfe-9b88-85f3c606be5e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ThirdPartyReinsurersMember_019c540a-9e2a-7b82-b96a-54b2ceed5809" xlink:href="nmih-20260331.xsd#nmih_ThirdPartyReinsurersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7cfe-9b88-85f3c606be5e" xlink:to="loc_nmih_ThirdPartyReinsurersMember_019c540a-9e2a-7b82-b96a-54b2ceed5809" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsuranceContractAxis_019c540a-9e2a-778c-8c3b-a6fe7070441a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReinsuranceContractAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-76f0-be40-3c5c296dbe94" xlink:to="loc_us-gaap_ReinsuranceContractAxis_019c540a-9e2a-778c-8c3b-a6fe7070441a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_RiskInForceCeded_019c540a-9e2a-7dc1-a450-564390298878" xlink:href="nmih-20260331.xsd#nmih_RiskInForceCeded"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:to="loc_nmih_RiskInForceCeded_019c540a-9e2a-7dc1-a450-564390298878" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededPremiumsEarned_019c540a-9e2a-7b67-8ac9-3735f65224b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededPremiumsEarned"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:to="loc_us-gaap_CededPremiumsEarned_019c540a-9e2a-7b67-8ac9-3735f65224b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceCededClaimsAndClaimsExpenses_019cf752-553f-767a-a945-19ad1e0b1a0c" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceCededClaimsAndClaimsExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:to="loc_nmih_ReinsuranceCededClaimsAndClaimsExpenses_019cf752-553f-767a-a945-19ad1e0b1a0c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceCedingCommissionEarned_019c540a-9e2a-7fab-b1ed-5958aac73924" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceCedingCommissionEarned"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:to="loc_nmih_ReinsuranceCedingCommissionEarned_019c540a-9e2a-7fab-b1ed-5958aac73924" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceProfitCommission_019c540a-9e2a-7782-b3e8-75235e866e0f" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceProfitCommission"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:to="loc_nmih_ReinsuranceProfitCommission_019c540a-9e2a-7782-b3e8-75235e866e0f" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/ReinsuranceScheduleofAmountsRelatedtoQSRTransactionsDetails_1" xlink:type="simple" xlink:href="nmih-20260331.xsd#ReinsuranceScheduleofAmountsRelatedtoQSRTransactionsDetails_1"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/ReinsuranceScheduleofAmountsRelatedtoQSRTransactionsDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-76f0-be40-3c5c296dbe94" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:to="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-76f0-be40-3c5c296dbe94" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7cdb-ada8-4e26c4d8f930" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CededCreditRiskTable_019c540a-9e2a-76f0-be40-3c5c296dbe94" xlink:to="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7cdb-ada8-4e26c4d8f930" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7cdb-ada8-4e26c4d8f930_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7cdb-ada8-4e26c4d8f930" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7cdb-ada8-4e26c4d8f930_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7cfe-9b88-85f3c606be5e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededCreditRiskReinsurerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CededCreditRiskAxis_019c540a-9e2a-7cdb-ada8-4e26c4d8f930" xlink:to="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7cfe-9b88-85f3c606be5e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ThirdPartyReinsurersMember_019c540a-9e2a-7b82-b96a-54b2ceed5809" xlink:href="nmih-20260331.xsd#nmih_ThirdPartyReinsurersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskReinsurerDomain_019c540a-9e2a-7cfe-9b88-85f3c606be5e" xlink:to="loc_nmih_ThirdPartyReinsurersMember_019c540a-9e2a-7b82-b96a-54b2ceed5809" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_RiskInForceCeded_019c540a-9e2a-7dc1-a450-564390298878" xlink:href="nmih-20260331.xsd#nmih_RiskInForceCeded"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:to="loc_nmih_RiskInForceCeded_019c540a-9e2a-7dc1-a450-564390298878" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CededPremiumsEarned_019c540a-9e2a-7b67-8ac9-3735f65224b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CededPremiumsEarned"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:to="loc_us-gaap_CededPremiumsEarned_019c540a-9e2a-7b67-8ac9-3735f65224b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceCededClaimsAndClaimsExpenses_019cf752-553f-767a-a945-19ad1e0b1a0c" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceCededClaimsAndClaimsExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:to="loc_nmih_ReinsuranceCededClaimsAndClaimsExpenses_019cf752-553f-767a-a945-19ad1e0b1a0c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceCedingCommissionEarned_019c540a-9e2a-7fab-b1ed-5958aac73924" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceCedingCommissionEarned"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:to="loc_nmih_ReinsuranceCedingCommissionEarned_019c540a-9e2a-7fab-b1ed-5958aac73924" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceProfitCommission_019c540a-9e2a-7782-b3e8-75235e866e0f" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceProfitCommission"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CededCreditRiskLineItems_019c540a-9e2a-7d03-bf44-09306cd66f0d" xlink:to="loc_nmih_ReinsuranceProfitCommission_019c540a-9e2a-7782-b3e8-75235e866e0f" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/ReinsuranceScheduleofILNTransactionsandXOLTransactionsDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#ReinsuranceScheduleofILNTransactionsandXOLTransactionsDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/ReinsuranceScheduleofILNTransactionsandXOLTransactionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTitleOfIndividualAndByTypeOfDeferredCompensationTable_019c540a-9e2a-7262-9917-f2a8a4870b08" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfDeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTitleOfIndividualAndByTypeOfDeferredCompensationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:to="loc_us-gaap_ScheduleOfDeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTitleOfIndividualAndByTypeOfDeferredCompensationTable_019c540a-9e2a-7262-9917-f2a8a4870b08" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsuranceContractAxis_019c540a-9e2a-7128-a5b2-1e5d0d8ad6c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReinsuranceContractAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTitleOfIndividualAndByTypeOfDeferredCompensationTable_019c540a-9e2a-7262-9917-f2a8a4870b08" xlink:to="loc_us-gaap_ReinsuranceContractAxis_019c540a-9e2a-7128-a5b2-1e5d0d8ad6c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_InitialReinsuranceCoverage_019c540a-9e2a-7cb8-b1bf-5c4e37d26653" xlink:href="nmih-20260331.xsd#nmih_InitialReinsuranceCoverage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:to="loc_nmih_InitialReinsuranceCoverage_019c540a-9e2a-7cb8-b1bf-5c4e37d26653" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_CurrentReinsuranceCoverage_019c540a-9e2a-78c1-b089-bdcb8422016a" xlink:href="nmih-20260331.xsd#nmih_CurrentReinsuranceCoverage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:to="loc_nmih_CurrentReinsuranceCoverage_019c540a-9e2a-78c1-b089-bdcb8422016a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_InitialFirstLayerRetainedLoss_019c540a-9e2a-7a12-b9e0-1a5697de3172" xlink:href="nmih-20260331.xsd#nmih_InitialFirstLayerRetainedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:to="loc_nmih_InitialFirstLayerRetainedLoss_019c540a-9e2a-7a12-b9e0-1a5697de3172" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_CurrentFirstLayerRetainedLoss_019c540a-9e2a-7aeb-8553-bf0d5442fa37" xlink:href="nmih-20260331.xsd#nmih_CurrentFirstLayerRetainedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:to="loc_nmih_CurrentFirstLayerRetainedLoss_019c540a-9e2a-7aeb-8553-bf0d5442fa37" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceCoveragePercentageOfPopulationWithProductionPriorToPeriodStartDate_019c540a-9e2a-7896-bcbd-8e2273099f4a" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceCoveragePercentageOfPopulationWithProductionPriorToPeriodStartDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:to="loc_nmih_ReinsuranceCoveragePercentageOfPopulationWithProductionPriorToPeriodStartDate_019c540a-9e2a-7896-bcbd-8e2273099f4a" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/ReinsuranceScheduleofILNTransactionsandXOLTransactionsDetails_1" xlink:type="simple" xlink:href="nmih-20260331.xsd#ReinsuranceScheduleofILNTransactionsandXOLTransactionsDetails_1"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/ReinsuranceScheduleofILNTransactionsandXOLTransactionsDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTitleOfIndividualAndByTypeOfDeferredCompensationTable_019c540a-9e2a-7262-9917-f2a8a4870b08" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfDeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTitleOfIndividualAndByTypeOfDeferredCompensationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:to="loc_us-gaap_ScheduleOfDeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTitleOfIndividualAndByTypeOfDeferredCompensationTable_019c540a-9e2a-7262-9917-f2a8a4870b08" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_InitialReinsuranceCoverage_019c540a-9e2a-7cb8-b1bf-5c4e37d26653" xlink:href="nmih-20260331.xsd#nmih_InitialReinsuranceCoverage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:to="loc_nmih_InitialReinsuranceCoverage_019c540a-9e2a-7cb8-b1bf-5c4e37d26653" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_CurrentReinsuranceCoverage_019c540a-9e2a-78c1-b089-bdcb8422016a" xlink:href="nmih-20260331.xsd#nmih_CurrentReinsuranceCoverage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:to="loc_nmih_CurrentReinsuranceCoverage_019c540a-9e2a-78c1-b089-bdcb8422016a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_InitialFirstLayerRetainedLoss_019c540a-9e2a-7a12-b9e0-1a5697de3172" xlink:href="nmih-20260331.xsd#nmih_InitialFirstLayerRetainedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:to="loc_nmih_InitialFirstLayerRetainedLoss_019c540a-9e2a-7a12-b9e0-1a5697de3172" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_CurrentFirstLayerRetainedLoss_019c540a-9e2a-7aeb-8553-bf0d5442fa37" xlink:href="nmih-20260331.xsd#nmih_CurrentFirstLayerRetainedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:to="loc_nmih_CurrentFirstLayerRetainedLoss_019c540a-9e2a-7aeb-8553-bf0d5442fa37" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReinsuranceCoveragePercentageOfPopulationWithProductionPriorToPeriodStartDate_019c540a-9e2a-7896-bcbd-8e2273099f4a" xlink:href="nmih-20260331.xsd#nmih_ReinsuranceCoveragePercentageOfPopulationWithProductionPriorToPeriodStartDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems_019c540a-9e2a-7605-baa2-6d94b2759533" xlink:to="loc_nmih_ReinsuranceCoveragePercentageOfPopulationWithProductionPriorToPeriodStartDate_019c540a-9e2a-7896-bcbd-8e2273099f4a" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/ReservesforInsuranceClaimsandClaimExpensesNarrativeDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#ReservesforInsuranceClaimsandClaimExpensesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/ReservesforInsuranceClaimsandClaimExpensesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseTable_019c540a-9e2a-78db-a585-71535418d8fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseTable_019c540a-9e2a-78db-a585-71535418d8fe" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReinsuranceContractAxis_019c540a-9e2a-771a-947f-76e4fd9064ca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReinsuranceContractAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseTable_019c540a-9e2a-78db-a585-71535418d8fe" xlink:to="loc_us-gaap_ReinsuranceContractAxis_019c540a-9e2a-771a-947f-76e4fd9064ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense_019c540a-9e2a-7de7-b216-7e61b73d5e22" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense_019c540a-9e2a-7de7-b216-7e61b73d5e22" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityforPaidClaimsandClaimsAdjustmentExpenseNumberofPaidClaims_019c540a-9e2a-7307-97e5-afdcad358440" xlink:href="nmih-20260331.xsd#nmih_LiabilityforPaidClaimsandClaimsAdjustmentExpenseNumberofPaidClaims"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityforPaidClaimsandClaimsAdjustmentExpenseNumberofPaidClaims_019c540a-9e2a-7307-97e5-afdcad358440" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsurance_019c540a-9e2a-7963-8563-f2e8de8192cb" xlink:href="nmih-20260331.xsd#nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsurance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsurance_019c540a-9e2a-7963-8563-f2e8de8192cb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityForPaidClaimsAndClaimsAdjustmentExpenseNumberOfCoveredClaimsIncludedInNumberOfPaidClaims_019c540a-9e2a-7a8f-9ca4-8ad8353c1237" xlink:href="nmih-20260331.xsd#nmih_LiabilityForPaidClaimsAndClaimsAdjustmentExpenseNumberOfCoveredClaimsIncludedInNumberOfPaidClaims"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityForPaidClaimsAndClaimsAdjustmentExpenseNumberOfCoveredClaimsIncludedInNumberOfPaidClaims_019c540a-9e2a-7a8f-9ca4-8ad8353c1237" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsuranceAmountUnderQSRTransaction_019c540a-9e2a-736b-901b-9a5caa22cc88" xlink:href="nmih-20260331.xsd#nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsuranceAmountUnderQSRTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsuranceAmountUnderQSRTransaction_019c540a-9e2a-736b-901b-9a5caa22cc88" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefault_019c540a-9e2a-78de-a60a-731fc1841a2b" xlink:href="nmih-20260331.xsd#nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefault"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefault_019c540a-9e2a-78de-a60a-731fc1841a2b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefaultPercentOfTotalPoliciesInForce_019c540a-9e2a-7f9b-b618-65450c0ffcf8" xlink:href="nmih-20260331.xsd#nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefaultPercentOfTotalPoliciesInForce"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefaultPercentOfTotalPoliciesInForce_019c540a-9e2a-7f9b-b618-65450c0ffcf8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsTotalPoliciesInForce_019c540a-9e2a-71d0-93eb-0f9c974b84eb" xlink:href="nmih-20260331.xsd#nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsTotalPoliciesInForce"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsTotalPoliciesInForce_019c540a-9e2a-71d0-93eb-0f9c974b84eb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_PriorYearClaimsAndClaimsAdjustmentExpenseFavorableDevelopmentOnPriorYearDefaults_019c540a-9e2a-7a82-9cab-d3fc5a65a9ce" xlink:href="nmih-20260331.xsd#nmih_PriorYearClaimsAndClaimsAdjustmentExpenseFavorableDevelopmentOnPriorYearDefaults"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_PriorYearClaimsAndClaimsAdjustmentExpenseFavorableDevelopmentOnPriorYearDefaults_019c540a-9e2a-7a82-9cab-d3fc5a65a9ce" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityforPriorYearClaimsandClaimsAdjustmentExpense_019c540a-9e2a-7400-b892-2ae9a85e3f49" xlink:href="nmih-20260331.xsd#nmih_LiabilityforPriorYearClaimsandClaimsAdjustmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityforPriorYearClaimsandClaimsAdjustmentExpense_019c540a-9e2a-7400-b892-2ae9a85e3f49" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/ReservesforInsuranceClaimsandClaimExpensesNarrativeDetails_1" xlink:type="simple" xlink:href="nmih-20260331.xsd#ReservesforInsuranceClaimsandClaimExpensesNarrativeDetails_1"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/ReservesforInsuranceClaimsandClaimExpensesNarrativeDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseTable_019c540a-9e2a-78db-a585-71535418d8fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseTable_019c540a-9e2a-78db-a585-71535418d8fe" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense_019c540a-9e2a-7de7-b216-7e61b73d5e22" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense_019c540a-9e2a-7de7-b216-7e61b73d5e22" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityforPaidClaimsandClaimsAdjustmentExpenseNumberofPaidClaims_019c540a-9e2a-7307-97e5-afdcad358440" xlink:href="nmih-20260331.xsd#nmih_LiabilityforPaidClaimsandClaimsAdjustmentExpenseNumberofPaidClaims"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityforPaidClaimsandClaimsAdjustmentExpenseNumberofPaidClaims_019c540a-9e2a-7307-97e5-afdcad358440" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsurance_019c540a-9e2a-7963-8563-f2e8de8192cb" xlink:href="nmih-20260331.xsd#nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsurance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsurance_019c540a-9e2a-7963-8563-f2e8de8192cb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityForPaidClaimsAndClaimsAdjustmentExpenseNumberOfCoveredClaimsIncludedInNumberOfPaidClaims_019c540a-9e2a-7a8f-9ca4-8ad8353c1237" xlink:href="nmih-20260331.xsd#nmih_LiabilityForPaidClaimsAndClaimsAdjustmentExpenseNumberOfCoveredClaimsIncludedInNumberOfPaidClaims"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityForPaidClaimsAndClaimsAdjustmentExpenseNumberOfCoveredClaimsIncludedInNumberOfPaidClaims_019c540a-9e2a-7a8f-9ca4-8ad8353c1237" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsuranceAmountUnderQSRTransaction_019c540a-9e2a-736b-901b-9a5caa22cc88" xlink:href="nmih-20260331.xsd#nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsuranceAmountUnderQSRTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseClaimsPaidIncludingAmountsCoveredByInsuranceAmountUnderQSRTransaction_019c540a-9e2a-736b-901b-9a5caa22cc88" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefault_019c540a-9e2a-78de-a60a-731fc1841a2b" xlink:href="nmih-20260331.xsd#nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefault"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefault_019c540a-9e2a-78de-a60a-731fc1841a2b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefaultPercentOfTotalPoliciesInForce_019c540a-9e2a-7f9b-b618-65450c0ffcf8" xlink:href="nmih-20260331.xsd#nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefaultPercentOfTotalPoliciesInForce"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsNumberOfPrimaryLoansInDefaultPercentOfTotalPoliciesInForce_019c540a-9e2a-7f9b-b618-65450c0ffcf8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsTotalPoliciesInForce_019c540a-9e2a-71d0-93eb-0f9c974b84eb" xlink:href="nmih-20260331.xsd#nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsTotalPoliciesInForce"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsTotalPoliciesInForce_019c540a-9e2a-71d0-93eb-0f9c974b84eb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_PriorYearClaimsAndClaimsAdjustmentExpenseFavorableDevelopmentOnPriorYearDefaults_019c540a-9e2a-7a82-9cab-d3fc5a65a9ce" xlink:href="nmih-20260331.xsd#nmih_PriorYearClaimsAndClaimsAdjustmentExpenseFavorableDevelopmentOnPriorYearDefaults"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_PriorYearClaimsAndClaimsAdjustmentExpenseFavorableDevelopmentOnPriorYearDefaults_019c540a-9e2a-7a82-9cab-d3fc5a65a9ce" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_LiabilityforPriorYearClaimsandClaimsAdjustmentExpense_019c540a-9e2a-7400-b892-2ae9a85e3f49" xlink:href="nmih-20260331.xsd#nmih_LiabilityforPriorYearClaimsandClaimsAdjustmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems_019c540a-9e2a-7147-b2c0-057ffc81e977" xlink:to="loc_nmih_LiabilityforPriorYearClaimsandClaimsAdjustmentExpense_019c540a-9e2a-7400-b892-2ae9a85e3f49" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/SegmentReportingDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#SegmentReportingDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/SegmentReportingDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019c540a-9e2a-7584-aa44-8c5a6c5085f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c540a-9e2a-7d0e-92c4-4d2b71f63255" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c540a-9e2a-7584-aa44-8c5a6c5085f7" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c540a-9e2a-7d0e-92c4-4d2b71f63255" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019c540a-9e2a-775d-9362-d05295899d74" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c540a-9e2a-7d0e-92c4-4d2b71f63255" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019c540a-9e2a-775d-9362-d05295899d74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019c540a-9e2a-775d-9362-d05295899d74_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019c540a-9e2a-775d-9362-d05295899d74" xlink:to="loc_us-gaap_SegmentDomain_019c540a-9e2a-775d-9362-d05295899d74_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019c540a-9e2a-700e-87dd-609d9146d313" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019c540a-9e2a-775d-9362-d05295899d74" xlink:to="loc_us-gaap_SegmentDomain_019c540a-9e2a-700e-87dd-609d9146d313" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_MortgageInsuranceSegmentMember_019c540a-9e2a-741e-b840-573236e3074b" xlink:href="nmih-20260331.xsd#nmih_MortgageInsuranceSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019c540a-9e2a-700e-87dd-609d9146d313" xlink:to="loc_nmih_MortgageInsuranceSegmentMember_019c540a-9e2a-741e-b840-573236e3074b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_019cf76e-fd31-70f6-ae7d-8048adbcdfa0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c540a-9e2a-7584-aa44-8c5a6c5085f7" xlink:to="loc_us-gaap_NumberOfReportableSegments_019cf76e-fd31-70f6-ae7d-8048adbcdfa0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInterestAndDividend_019c540a-9e2a-78a9-a461-4375562e9bd0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeInterestAndDividend"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c540a-9e2a-7584-aa44-8c5a6c5085f7" xlink:to="loc_us-gaap_InvestmentIncomeInterestAndDividend_019c540a-9e2a-78a9-a461-4375562e9bd0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAmortizationAndAccretionNet_019c540a-9e2a-7cff-9234-f75a6363e279" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationAmortizationAndAccretionNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c540a-9e2a-7584-aa44-8c5a6c5085f7" xlink:to="loc_us-gaap_DepreciationAmortizationAndAccretionNet_019c540a-9e2a-7cff-9234-f75a6363e279" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/RegulatoryInformationNarrativeDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#RegulatoryInformationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/RegulatoryInformationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatutoryAccountingPracticesLineItems_019c540a-9e2a-7875-942c-e2a4788915cd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatutoryAccountingPracticesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatutoryAccountingPracticesTable_019c540a-9e2a-7676-b909-9c6f91e8f657" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatutoryAccountingPracticesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatutoryAccountingPracticesLineItems_019c540a-9e2a-7875-942c-e2a4788915cd" xlink:to="loc_us-gaap_StatutoryAccountingPracticesTable_019c540a-9e2a-7676-b909-9c6f91e8f657" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019c540a-9e2a-7b70-9739-132e027f8ba0" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatutoryAccountingPracticesTable_019c540a-9e2a-7676-b909-9c6f91e8f657" xlink:to="loc_dei_LegalEntityAxis_019c540a-9e2a-7b70-9739-132e027f8ba0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019c540a-9e2a-7b70-9739-132e027f8ba0_default" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019c540a-9e2a-7b70-9739-132e027f8ba0" xlink:to="loc_dei_EntityDomain_019c540a-9e2a-7b70-9739-132e027f8ba0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019c540a-9e2a-716f-a9f2-5aae5d8c34e8" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019c540a-9e2a-7b70-9739-132e027f8ba0" xlink:to="loc_dei_EntityDomain_019c540a-9e2a-716f-a9f2-5aae5d8c34e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_NMICAndReOneCombinedMember_019c540a-9e2a-7bfb-a498-36806a743b3b" xlink:href="nmih-20260331.xsd#nmih_NMICAndReOneCombinedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019c540a-9e2a-716f-a9f2-5aae5d8c34e8" xlink:to="loc_nmih_NMICAndReOneCombinedMember_019c540a-9e2a-7bfb-a498-36806a743b3b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_ReOneMember_019c540a-9e2a-7563-8ad2-bd4091ee421e" xlink:href="nmih-20260331.xsd#nmih_ReOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019c540a-9e2a-716f-a9f2-5aae5d8c34e8" xlink:to="loc_nmih_ReOneMember_019c540a-9e2a-7563-8ad2-bd4091ee421e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_019c540a-9e2a-77d9-bfe5-bc97c644e514" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatutoryAccountingPracticesTable_019c540a-9e2a-7676-b909-9c6f91e8f657" xlink:to="loc_srt_StatementScenarioAxis_019c540a-9e2a-77d9-bfe5-bc97c644e514" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019c540a-9e2a-77d9-bfe5-bc97c644e514_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_019c540a-9e2a-77d9-bfe5-bc97c644e514" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019c540a-9e2a-77d9-bfe5-bc97c644e514_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019c540a-9e2a-7bf8-a483-730764432541" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_019c540a-9e2a-77d9-bfe5-bc97c644e514" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019c540a-9e2a-7bf8-a483-730764432541" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioForecastMember_019c540a-9e2a-79c2-b659-fe9427da60ec" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioForecastMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ScenarioUnspecifiedDomain_019c540a-9e2a-7bf8-a483-730764432541" xlink:to="loc_srt_ScenarioForecastMember_019c540a-9e2a-79c2-b659-fe9427da60ec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount_019c540a-9e2a-7670-b521-2e46c1eb8de6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatutoryAccountingPracticesLineItems_019c540a-9e2a-7875-942c-e2a4788915cd" xlink:to="loc_us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount_019c540a-9e2a-7670-b521-2e46c1eb8de6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_StatutoryAccountingPracticesStatutoryCapitalAndSurplusIncludingContingencyReserve_019c540a-9e2a-7825-94e2-0bde1c1998ad" xlink:href="nmih-20260331.xsd#nmih_StatutoryAccountingPracticesStatutoryCapitalAndSurplusIncludingContingencyReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatutoryAccountingPracticesLineItems_019c540a-9e2a-7875-942c-e2a4788915cd" xlink:to="loc_nmih_StatutoryAccountingPracticesStatutoryCapitalAndSurplusIncludingContingencyReserve_019c540a-9e2a-7825-94e2-0bde1c1998ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatutoryAccountingPracticesStatutoryAmountAvailableForDividendPayments_019c540a-9e2a-7b0a-9b41-0523d92ae276" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatutoryAccountingPracticesStatutoryAmountAvailableForDividendPayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatutoryAccountingPracticesLineItems_019c540a-9e2a-7875-942c-e2a4788915cd" xlink:to="loc_us-gaap_StatutoryAccountingPracticesStatutoryAmountAvailableForDividendPayments_019c540a-9e2a-7b0a-9b41-0523d92ae276" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.nationalmi.com/role/RegulatoryInformationScheduleofStatutoryNetIncomelossSurplusContingencyReserveandRisktoCapitalRatioDetails" xlink:type="simple" xlink:href="nmih-20260331.xsd#RegulatoryInformationScheduleofStatutoryNetIncomelossSurplusContingencyReserveandRisktoCapitalRatioDetails"/>
  <link:definitionLink xlink:role="http://www.nationalmi.com/role/RegulatoryInformationScheduleofStatutoryNetIncomelossSurplusContingencyReserveandRisktoCapitalRatioDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatutoryAccountingPracticesLineItems_019c540a-9e2a-77a1-962a-12bbf5598c31" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatutoryAccountingPracticesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatutoryAccountingPracticesTable_019c540a-9e2a-7ad0-977e-4fbfe8372922" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatutoryAccountingPracticesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatutoryAccountingPracticesLineItems_019c540a-9e2a-77a1-962a-12bbf5598c31" xlink:to="loc_us-gaap_StatutoryAccountingPracticesTable_019c540a-9e2a-7ad0-977e-4fbfe8372922" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019c540a-9e2a-752f-9d50-944802ccf36d" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatutoryAccountingPracticesTable_019c540a-9e2a-7ad0-977e-4fbfe8372922" xlink:to="loc_dei_LegalEntityAxis_019c540a-9e2a-752f-9d50-944802ccf36d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019c540a-9e2a-752f-9d50-944802ccf36d_default" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019c540a-9e2a-752f-9d50-944802ccf36d" xlink:to="loc_dei_EntityDomain_019c540a-9e2a-752f-9d50-944802ccf36d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019c540a-9e2a-7427-bde6-5cc8958ca1c0" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019c540a-9e2a-752f-9d50-944802ccf36d" xlink:to="loc_dei_EntityDomain_019c540a-9e2a-7427-bde6-5cc8958ca1c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_NMICAndReOneCombinedMember_019c540a-9e2a-7f4a-ae32-7c9203ea2884" xlink:href="nmih-20260331.xsd#nmih_NMICAndReOneCombinedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019c540a-9e2a-7427-bde6-5cc8958ca1c0" xlink:to="loc_nmih_NMICAndReOneCombinedMember_019c540a-9e2a-7f4a-ae32-7c9203ea2884" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance_019c540a-9e2a-7632-93a8-4ee07308e8cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatutoryAccountingPracticesLineItems_019c540a-9e2a-77a1-962a-12bbf5598c31" xlink:to="loc_us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance_019c540a-9e2a-7632-93a8-4ee07308e8cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_StatutoryAccountingPracticesContingencyReserve_019c540a-9e2a-7912-8a4f-748f8ffa6e87" xlink:href="nmih-20260331.xsd#nmih_StatutoryAccountingPracticesContingencyReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatutoryAccountingPracticesLineItems_019c540a-9e2a-77a1-962a-12bbf5598c31" xlink:to="loc_nmih_StatutoryAccountingPracticesContingencyReserve_019c540a-9e2a-7912-8a4f-748f8ffa6e87" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_StatutoryAccountingPracticesStatutoryCapitalAndSurplusIncludingContingencyReserve_019c540a-9e2a-75ec-b438-8bba83d86d3d" xlink:href="nmih-20260331.xsd#nmih_StatutoryAccountingPracticesStatutoryCapitalAndSurplusIncludingContingencyReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatutoryAccountingPracticesLineItems_019c540a-9e2a-77a1-962a-12bbf5598c31" xlink:to="loc_nmih_StatutoryAccountingPracticesStatutoryCapitalAndSurplusIncludingContingencyReserve_019c540a-9e2a-75ec-b438-8bba83d86d3d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_nmih_RiskToCapitalRatioRegulatoryActual_019c540a-9e2a-7e50-9c2b-099b376369b2" xlink:href="nmih-20260331.xsd#nmih_RiskToCapitalRatioRegulatoryActual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatutoryAccountingPracticesLineItems_019c540a-9e2a-77a1-962a-12bbf5598c31" xlink:to="loc_nmih_RiskToCapitalRatioRegulatoryActual_019c540a-9e2a-7e50-9c2b-099b376369b2" xlink:type="arc" order="3"/>
  </link:definitionLink>
</link:linkbase>
