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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019f1524-c8a0-73d5-a738-7e63cf3adf70,g:f7409f97-606d-45ae-801e-fac6b671cf0e-->
<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.whitemountains.com/role/Cover" xlink:type="simple" xlink:href="wtm-20260630.xsd#Cover"/>
  <link:definitionLink xlink:role="http://www.whitemountains.com/role/Cover" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_019f15a3-899e-7ebb-a164-8102d969bb7f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_019f15a3-899e-7fc7-be4f-9984bc878e39" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_019f15a3-899e-7ebb-a164-8102d969bb7f" xlink:to="loc_dei_DocumentInformationTable_019f15a3-899e-7fc7-be4f-9984bc878e39" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019f15a5-0ff0-7700-a58f-53c3ded21346" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_019f15a3-899e-7fc7-be4f-9984bc878e39" xlink:to="loc_dei_LegalEntityAxis_019f15a5-0ff0-7700-a58f-53c3ded21346" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f15a5-0ff0-7700-a58f-53c3ded21346_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019f15a5-0ff0-7700-a58f-53c3ded21346" xlink:to="loc_dei_EntityDomain_019f15a5-0ff0-7700-a58f-53c3ded21346_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f15a5-0ff0-7e40-8557-1cbb5c0b4008" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019f15a5-0ff0-7700-a58f-53c3ded21346" xlink:to="loc_dei_EntityDomain_019f15a5-0ff0-7e40-8557-1cbb5c0b4008" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_wtm_EBP001Member_019f15a5-1c39-7b92-b8b8-db7f66059a9f" xlink:href="wtm-20260630.xsd#wtm_EBP001Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f15a5-0ff0-7e40-8557-1cbb5c0b4008" xlink:to="loc_wtm_EBP001Member_019f15a5-1c39-7b92-b8b8-db7f66059a9f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_019f15a5-e137-730e-b4be-5dc4a48a802b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f15a3-899e-7ebb-a164-8102d969bb7f" xlink:to="loc_dei_DocumentType_019f15a5-e137-730e-b4be-5dc4a48a802b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_019f15a6-9520-7b26-9b03-e367f837d961" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f15a3-899e-7ebb-a164-8102d969bb7f" xlink:to="loc_dei_EntityRegistrantName_019f15a6-9520-7b26-9b03-e367f837d961" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_019f15a3-899e-7079-b826-ba80e7d82870" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f15a3-899e-7ebb-a164-8102d969bb7f" xlink:to="loc_dei_EntityCentralIndexKey_019f15a3-899e-7079-b826-ba80e7d82870" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_019f15a3-a863-7e31-9560-4d385c56a98a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f15a3-899e-7ebb-a164-8102d969bb7f" xlink:to="loc_dei_AmendmentFlag_019f15a3-a863-7e31-9560-4d385c56a98a" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.whitemountains.com/role/BasicFinancialInformation" xlink:type="simple" xlink:href="wtm-20260630.xsd#BasicFinancialInformation"/>
  <link:definitionLink xlink:role="http://www.whitemountains.com/role/BasicFinancialInformation" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b8-9b34-7888-9a2a-180a945111f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanTable_019f15b8-9b34-7935-87af-cd6fd3408ee3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b8-9b34-7888-9a2a-180a945111f4" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanTable_019f15b8-9b34-7935-87af-cd6fd3408ee3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019f15b8-9b34-775f-95b2-8fc034b9a884" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanTable_019f15b8-9b34-7935-87af-cd6fd3408ee3" xlink:to="loc_dei_LegalEntityAxis_019f15b8-9b34-775f-95b2-8fc034b9a884" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f15b8-9b34-775f-95b2-8fc034b9a884_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019f15b8-9b34-775f-95b2-8fc034b9a884" xlink:to="loc_dei_EntityDomain_019f15b8-9b34-775f-95b2-8fc034b9a884_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f15b8-9b34-7f7b-bbdb-d8f13953b09c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019f15b8-9b34-775f-95b2-8fc034b9a884" xlink:to="loc_dei_EntityDomain_019f15b8-9b34-7f7b-bbdb-d8f13953b09c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_wtm_EBP001Member_019f15b8-9b34-72e5-ae78-3a8f292d4fc9" xlink:href="wtm-20260630.xsd#wtm_EBP001Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f15b8-9b34-7f7b-bbdb-d8f13953b09c" xlink:to="loc_wtm_EBP001Member_019f15b8-9b34-72e5-ae78-3a8f292d4fc9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanTextBlock_019f15b8-9b34-7fcf-9836-f7442f45f7c1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanTextBlock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b8-9b34-7888-9a2a-180a945111f4" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanTextBlock_019f15b8-9b34-7fcf-9836-f7442f45f7c1" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.whitemountains.com/role/ScheduleIForm5500" xlink:type="simple" xlink:href="wtm-20260630.xsd#ScheduleIForm5500"/>
  <link:definitionLink xlink:role="http://www.whitemountains.com/role/ScheduleIForm5500" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15b5-185f-7c9d-9fc7-971a2a8f2234" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500Table_019f15b5-185f-7d16-8dbd-744cc456cc21" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500Table"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15b5-185f-7c9d-9fc7-971a2a8f2234" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500Table_019f15b5-185f-7d16-8dbd-744cc456cc21" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019f15b6-f497-75e1-b737-78cf35fb960a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500Table_019f15b5-185f-7d16-8dbd-744cc456cc21" xlink:to="loc_dei_LegalEntityAxis_019f15b6-f497-75e1-b737-78cf35fb960a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f15b6-f497-75e1-b737-78cf35fb960a_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019f15b6-f497-75e1-b737-78cf35fb960a" xlink:to="loc_dei_EntityDomain_019f15b6-f497-75e1-b737-78cf35fb960a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f15b6-f498-7319-a0a6-e4a9b0aa2f6c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019f15b6-f497-75e1-b737-78cf35fb960a" xlink:to="loc_dei_EntityDomain_019f15b6-f498-7319-a0a6-e4a9b0aa2f6c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_wtm_EBP001Member_019f15b6-f498-7def-a5ae-fb9f4b3d34ca" xlink:href="wtm-20260630.xsd#wtm_EBP001Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f15b6-f498-7319-a0a6-e4a9b0aa2f6c" xlink:to="loc_wtm_EBP001Member_019f15b6-f498-7def-a5ae-fb9f4b3d34ca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500TextBlock_019f15b5-185f-7f14-93eb-1390d5f9d9d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500TextBlock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15b5-185f-7c9d-9fc7-971a2a8f2234" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500TextBlock_019f15b5-185f-7f14-93eb-1390d5f9d9d9" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.whitemountains.com/role/ScheduleIForm5500Tables" xlink:type="simple" xlink:href="wtm-20260630.xsd#ScheduleIForm5500Tables"/>
  <link:definitionLink xlink:role="http://www.whitemountains.com/role/ScheduleIForm5500Tables" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15bb-7539-73a4-b49f-23ad426c2432" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500Table_019f15bb-7539-7292-8825-9a200f9e51e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500Table"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15bb-7539-73a4-b49f-23ad426c2432" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500Table_019f15bb-7539-7292-8825-9a200f9e51e9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019f15bb-7539-777e-a514-29f28a366cd1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500Table_019f15bb-7539-7292-8825-9a200f9e51e9" xlink:to="loc_dei_LegalEntityAxis_019f15bb-7539-777e-a514-29f28a366cd1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f15bb-7539-777e-a514-29f28a366cd1_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019f15bb-7539-777e-a514-29f28a366cd1" xlink:to="loc_dei_EntityDomain_019f15bb-7539-777e-a514-29f28a366cd1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f15bb-7539-77fb-a747-e55e49def830" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019f15bb-7539-777e-a514-29f28a366cd1" xlink:to="loc_dei_EntityDomain_019f15bb-7539-77fb-a747-e55e49def830" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_wtm_EBP001Member_019f15bd-1c0a-7fb5-a72e-8ca7bc1cadf8" xlink:href="wtm-20260630.xsd#wtm_EBP001Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f15bb-7539-77fb-a747-e55e49def830" xlink:to="loc_wtm_EBP001Member_019f15bd-1c0a-7fb5-a72e-8ca7bc1cadf8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500TableTextBlock_019f15bb-7539-731c-901a-1b24623e999e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500TableTextBlock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15bb-7539-73a4-b49f-23ad426c2432" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500TableTextBlock_019f15bb-7539-731c-901a-1b24623e999e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.whitemountains.com/role/BasicFinancialInformationDetails" xlink:type="simple" xlink:href="wtm-20260630.xsd#BasicFinancialInformationDetails"/>
  <link:definitionLink xlink:role="http://www.whitemountains.com/role/BasicFinancialInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanTable_019f15b4-d080-7e83-bc75-04f6ee9767bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanTable_019f15b4-d080-7e83-bc75-04f6ee9767bd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019f15b4-d080-7b29-a120-bbc3a3b13972" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanTable_019f15b4-d080-7e83-bc75-04f6ee9767bd" xlink:to="loc_dei_LegalEntityAxis_019f15b4-d080-7b29-a120-bbc3a3b13972" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f15b4-d080-7b29-a120-bbc3a3b13972_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019f15b4-d080-7b29-a120-bbc3a3b13972" xlink:to="loc_dei_EntityDomain_019f15b4-d080-7b29-a120-bbc3a3b13972_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f15b4-d080-7bb8-8367-65329123a8f6" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019f15b4-d080-7b29-a120-bbc3a3b13972" xlink:to="loc_dei_EntityDomain_019f15b4-d080-7bb8-8367-65329123a8f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_wtm_EBP001Member_019f15b4-d080-7538-ba33-abb6996042f4" xlink:href="wtm-20260630.xsd#wtm_EBP001Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f15b4-d080-7bb8-8367-65329123a8f6" xlink:to="loc_wtm_EBP001Member_019f15b4-d080-7538-ba33-abb6996042f4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecrease_019f15b4-d080-7ede-8412-3f105846f8e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecrease_019f15b4-d080-7ede-8412-3f105846f8e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecreaseForAdministrativeExpense_019f15b4-d080-703c-a2ea-3fde63c24f60" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecreaseForAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecreaseForAdministrativeExpense_019f15b4-d080-703c-a2ea-3fde63c24f60" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecreaseForPaymentToParticipant_019f15b4-d080-70b9-999d-12a9b3aded5d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecreaseForPaymentToParticipant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecreaseForPaymentToParticipant_019f15b4-d080-70b9-999d-12a9b3aded5d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_wtm_EBPChangeInNetAssetAvailableForBenefitDecreaseFromOtherExpenses_019f15b4-d080-70a6-92b7-1d5c2d530ead" xlink:href="wtm-20260630.xsd#wtm_EBPChangeInNetAssetAvailableForBenefitDecreaseFromOtherExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:to="loc_wtm_EBPChangeInNetAssetAvailableForBenefitDecreaseFromOtherExpenses_019f15b4-d080-70a6-92b7-1d5c2d530ead" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanNumberOfParticipants_019f15b4-d080-7e1a-94c1-76d7b044aad3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanNumberOfParticipants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanNumberOfParticipants_019f15b4-d080-7e1a-94c1-76d7b044aad3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanNetAssetAvailableForBenefit_019f15b4-d080-7ba2-b4f1-b42163418e02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanNetAssetAvailableForBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanNetAssetAvailableForBenefit_019f15b4-d080-7ba2-b4f1-b42163418e02" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseDecrease_019f15b4-d080-71c3-be66-811e9ee0cc4b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseDecrease_019f15b4-d080-71c3-be66-811e9ee0cc4b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncrease_019f15b4-d080-7f4c-add5-7722b9935729" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncrease_019f15b4-d080-7f4c-add5-7722b9935729" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseForEmployerContributionCash_019f15b4-d080-7778-acce-215df1ea269d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseForEmployerContributionCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseForEmployerContributionCash_019f15b4-d080-7778-acce-215df1ea269d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseForParticipantContribution_019f15b4-d080-74d6-ad42-2d61ba62bbb2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseForParticipantContribution"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseForParticipantContribution_019f15b4-d080-74d6-ad42-2d61ba62bbb2" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseDecreaseForIncomeLossOnInvestment_019f15b4-d080-749c-bbf2-d196504e65e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseDecreaseForIncomeLossOnInvestment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanDescriptionOfPlanLineItems_019f15b4-d080-7beb-b5ad-7f16a9aa025a" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseDecreaseForIncomeLossOnInvestment_019f15b4-d080-749c-bbf2-d196504e65e2" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.whitemountains.com/role/ScheduleIForm5500Details" xlink:type="simple" xlink:href="wtm-20260630.xsd#ScheduleIForm5500Details"/>
  <link:definitionLink xlink:role="http://www.whitemountains.com/role/ScheduleIForm5500Details" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500Table_019f15a5-74cc-7ceb-ad31-2100d2383307" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500Table"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500Table_019f15a5-74cc-7ceb-ad31-2100d2383307" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019f15a5-74cc-74fe-a71c-92c185e3b3a4" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500Table_019f15a5-74cc-7ceb-ad31-2100d2383307" xlink:to="loc_dei_LegalEntityAxis_019f15a5-74cc-74fe-a71c-92c185e3b3a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f15a5-74cc-74fe-a71c-92c185e3b3a4_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019f15a5-74cc-74fe-a71c-92c185e3b3a4" xlink:to="loc_dei_EntityDomain_019f15a5-74cc-74fe-a71c-92c185e3b3a4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f15a5-74cc-707b-a57c-9ca0d07e47f3" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019f15a5-74cc-74fe-a71c-92c185e3b3a4" xlink:to="loc_dei_EntityDomain_019f15a5-74cc-707b-a57c-9ca0d07e47f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_wtm_EBP001Member_019f15a6-2943-7599-9fdc-28dd96167d5e" xlink:href="wtm-20260630.xsd#wtm_EBP001Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f15a5-74cc-707b-a57c-9ca0d07e47f3" xlink:to="loc_wtm_EBP001Member_019f15a6-2943-7599-9fdc-28dd96167d5e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionAsset_019f15a9-23bc-7dd9-a9eb-db2137b7cdf7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionAsset_019f15a9-23bc-7dd9-a9eb-db2137b7cdf7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionLiability_019f15a9-6396-7531-895e-3cf26c91ee07" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionLiability_019f15a9-6396-7531-895e-3cf26c91ee07" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanNetAssetAvailableForBenefit_019f15a9-90e1-71a4-b47b-52adcb23a520" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanNetAssetAvailableForBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanNetAssetAvailableForBenefit_019f15a9-90e1-71a4-b47b-52adcb23a520" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionEmployerContribution_019f15a5-74cc-724c-80c4-a10abdbaf270" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionEmployerContribution"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionEmployerContribution_019f15a5-74cc-724c-80c4-a10abdbaf270" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionParticipantContribution_019f15a7-0772-795f-ad63-87c938f0694b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionParticipantContribution"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionParticipantContribution_019f15a7-0772-795f-ad63-87c938f0694b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionOtherContribution_019f15a7-9b39-7552-ba71-e0e2d82181ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionOtherContribution"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionOtherContribution_019f15a7-9b39-7552-ba71-e0e2d82181ff" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionOtherIncome_019f15a8-1d8e-71b4-93b8-2bdeccc18bb5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionOtherIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionOtherIncome_019f15a8-1d8e-71b4-93b8-2bdeccc18bb5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionTotalIncome_019f15a8-6db4-7153-af6a-4d876b961af8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionTotalIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionTotalIncome_019f15a8-6db4-7153-af6a-4d876b961af8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionPaymentToParticipant_019f15aa-7243-72d9-946e-c046776621ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionPaymentToParticipant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionPaymentToParticipant_019f15aa-7243-72d9-946e-c046776621ea" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionTotalAdministrativeExpense_019f15aa-f82d-7200-8f8f-9400d61df072" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionTotalAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionTotalAdministrativeExpense_019f15aa-f82d-7200-8f8f-9400d61df072" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionTotalExpense_019f15ab-33d7-7f69-a08c-9a1d6a24460d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionTotalExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionTotalExpense_019f15ab-33d7-7f69-a08c-9a1d6a24460d" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionNetIncomeLoss_019f15ab-645f-7fd1-824c-7571bd1be30b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionNetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionNetIncomeLoss_019f15ab-645f-7fd1-824c-7571bd1be30b" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_wtm_EBPForm5500CaptionEmployerSecurities_019f15ac-b58d-72e6-afae-840d9c061e6e" xlink:href="wtm-20260630.xsd#wtm_EBPForm5500CaptionEmployerSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_wtm_EBPForm5500CaptionEmployerSecurities_019f15ac-b58d-72e6-afae-840d9c061e6e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionParticipantLoan_019f15ac-382d-7298-ba06-385cf30c9594" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionParticipantLoan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionParticipantLoan_019f15ac-382d-7298-ba06-385cf30c9594" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_wtm_EBPForm5500Caption_019f15ad-24e5-7747-bbcf-3b0d77430283" xlink:href="wtm-20260630.xsd#wtm_EBPForm5500Caption"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_wtm_EBPForm5500Caption_019f15ad-24e5-7747-bbcf-3b0d77430283" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_019f15ad-4ff1-7d45-a797-fd5f22b48d6c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_dei_EntityTaxIdentificationNumber_019f15ad-4ff1-7d45-a797-fd5f22b48d6c" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap-ebp_EmployeeBenefitPlanPlanNumber_019f15ad-84dd-7ec0-99bd-712966f7950e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/ebp/elts/us-gaap-ebp-2026.xsd#us-gaap-ebp_EmployeeBenefitPlanPlanNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500LineItems_019f15a5-74cc-71fc-ac21-e41007f72017" xlink:to="loc_us-gaap-ebp_EmployeeBenefitPlanPlanNumber_019f15ad-84dd-7ec0-99bd-712966f7950e" xlink:type="arc" order="16"/>
  </link:definitionLink>
</link:linkbase>
