8-K

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

July 18, 2025

Date of Report (Date of earliest event reported)

WHITE MOUNTAINS INSURANCE GROUP, LTD.

(Exact name of registrant as specified in its charter)

Bermuda<br><br>(State or other jurisdiction of<br><br>incorporation or organization)	1-8993<br><br>(Commission file number)	94-2708455<br><br>(I.R.S. Employer Identification No.)

23 South Main Street, Suite 3B, Hanover, New Hampshire 03755

(Address of principal executive offices)

(603) 640-2200

(Registrant’s telephone number, including area code)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

☐ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

☐ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

☐ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

☐ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

Title of each class	Trading Symbol(s)	Name of each exchange on which registered
Common Shares, par value $1.00 per share	WTM	New York Stock Exchange
	WTM.BH	Bermuda Stock Exchange

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933(§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).                                     Emerging growth company ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.      o

ITEM 8.01 Other Events.

On July 18, 2025, the Company closed its previously announced $150 million investment in BroadStreet Partners (“BroadStreet”). A copy of the press release jointly issued by BroadStreet, the Company and the other co-lead investors is furnished as Exhibit 99.1 to this report.

ITEM 9.01   Financial Statements and Exhibits.

(d) Exhibits

99.1wtmex991broadstreetpressre.htmPress Release, dated July 18, 2025

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.

	WHITE MOUNTAINS INSURANCE GROUP, LTD.
Date: July 18, 2025	By:	/s/   MICHAELA J. HILDRETH<br><br>Michaela J. Hildreth<br><br>Managing Director and<br><br>Chief Accounting Officer

3

Document

Exhibit 99.1

BroadStreet Partners Completes Strategic Investment

COLUMBUS, Ohio--(BUSINESS WIRE)--BroadStreet Partners (“BroadStreet” or the “Company”) today announced the successful closing of a strategic investment by an investor group led by Ethos Capital (“Ethos”), British Columbia Investment Management Corporation (“BCI”), and White Mountains Insurance Group, Ltd. (“White Mountains”), marking a significant milestone in the Company’s growth trajectory. Accounts advised by T. Rowe Price Investment Management, Inc. also participated. Ontario Teachers’ Pension Plan (“Ontario Teachers’”) will retain a significant co-control stake and continue its partnership with the Ethos-led investor group.

BroadStreet is a leading middle-market insurance brokerage providing commercial and personal property & casualty and employee benefits solutions. The Company partners with leading independent insurance agencies, known as Core Agency Partners, and supports them with M&A capabilities, capital solutions, and a comprehensive suite of resources and tools designed to accelerate organic growth. BroadStreet’s distinctive co-ownership model, commitment to innovation, and continued investment in digital transformation position its Core Agency Partners for sustained success and long-term growth.

Ardea Partners served as lead financial advisor to Ontario Teachers’ and BroadStreet, and RBC Capital Markets and BMO Capital Markets served as co-advisors. Latham & Watkins LLP and Torys LLP served as legal counsel to Ontario Teachers’ and BroadStreet. Kirkland & Ellis LLP served as legal counsel to Ethos Capital. Debevoise & Plimpton LLP served as legal counsel to BCI. Cravath, Swaine & Moore LLP served as legal counsel to White Mountains.

About BroadStreet Partners

BroadStreet Partners is an insurance brokerage company headquartered in Columbus, Ohio. The Company invests in select, entrepreneurial, high-performing independent agencies looking for capital support and partnership. With 30 Core Agency Partners, BroadStreet provides ownership opportunities for more than 800 agency professionals across the U.S. and Canada. www.broadstreetcorp.com

About Ethos Capital

Ethos Capital is a global investment firm that makes majority and minority investments in middle-market companies, primarily across North America and Europe. The firm’s Executive Partners collaborate with its portfolio companies, providing them with a network of resources and experience to strategically enhance operations and accelerate their growth. For more information, visit www.ethoscapital.com.

About BCI

British Columbia Investment Management Corporation (BCI) is one of Canada’s largest institutional investors, with C$295 billion in gross assets under management as of March 31, 2025. For 25 years, BCI has built its legacy on performance with purpose, helping its 32 public sector and institutional clients deliver on their commitments. From securing pensions to supporting communities, it’s investing that matters.

Headquartered in Victoria, British Columbia, and with teams spanning Vancouver, New York, London, and Mumbai, BCI puts patient capital to work across public and private markets globally.

Learn more on BCI.ca or connect on LinkedIn.

About White Mountains

White Mountains Insurance Group, Ltd. (NYSE: WTM) is a Bermuda-domiciled financial services holding company traded on the New York Stock Exchange and the Bermuda Stock Exchange under the symbols WTM and WTM.BH, respectively. Additional financial information and other items of interest are available at the company's website located at www.whitemountains.com.

About T. Rowe Price

T. Rowe Price (NASDAQ-GS: TROW) is a leading global asset management firm, entrusted with managing $1.68 trillion in client assets as of June 30, 2025, about two-thirds of which are retirement-related. Renowned for over 85 years of investment excellence, retirement leadership, and independent proprietary research, the firm leverages its longstanding expertise to ask better questions that can drive better investment decisions. Built on a culture of integrity and prioritizing client interests, T. Rowe Price empowers millions of investors worldwide to thrive amid evolving markets.

About Ontario Teachers’

Ontario Teachers' Pension Plan Board (Ontario Teachers') is a global investor with net assets of $266.3 billion as of December 31, 2024. Ontario Teachers’ is a fully funded defined benefit pension plan, and it invests in a broad array of asset classes to deliver retirement security for 343,000 working members and pensioners. For more information, visit otpp.com and follow on LinkedIn.

Contacts

Ethos Capital

Prosek Partners

Bea Broderick

pro-ethoscapital@prosek.com

BCI

Olga Petrycki

media@bci.ca

Ontario Teachers’

Dan Madge/James DeCosimo

media@otpp.com