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

August 26, 2026

Date of Report (Date of earliest event reported)

WHITE MOUNTAINS INSURANCE GROUP, LTD.

(Exact name of registrant as specified in its charter)

Bermuda (State or other jurisdiction of  incorporation or organization) 1-8993 (Commission file number) 94-2708455 (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 5.02 Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.

On August 26, 2026, the Board of Directors (the “Board”) of White Mountains Insurance Group, Ltd. (the “Company”) elected Stephen Klar as a director of the Company. In connection with Mr. Klar’s election, the size of the Board was increased from 10 to 11 persons. Mr. Klar will hold office as a director on the Board until the Company’s next annual meeting of shareholders and until his successor is elected and qualified or until his earlier resignation or removal.

Mr. Klar will receive compensation for his service on the Board in accordance with the Company’s standard compensatory arrangement for non-employee directors, including a pro-rated cash and common equity retainer for the remainder of the current year. A description of the Company’s non-employee director compensation appears under the caption “Director Compensation” in the Company’s Proxy Statement filed with the Securities and Exchange Commission on April 1, 2026.

There are no arrangements or understandings between Mr. Klar and any other persons pursuant to which he was elected as a director. Mr. Klar has no direct or indirect material interest in any transaction required to be disclosed pursuant to Item 404(a) of Regulation S-K.

Our press release announcing Mr. Klar’s election as director is attached as Exhibit 99.1 and incorporated by reference herein.

ITEM 9.01  Financial Statements and Exhibits.

(d) Exhibits

99.1    Press Release, dated as of August 27, 2026

104    Cover Page Interactive Data File (the cover page XBRL tags are embedded within the Inline XBRL document)

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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.
August 27, 2026	By:	/s/   MICHAELA J. HILDRETH            Michaela J. Hildreth       Managing Director and Chief Accounting Officer

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EX-99.1

Exhibit 99.1

PRESS RELEASE CONTACT: Rob Seelig (603) 640-2212

Stephen Klar Elected to the White Mountains Board

HAMILTON, Bermuda, August 27, 2026 /PRNewswire/ -- The Board of Directors of White Mountains Insurance Group, Ltd. (NYSE: WTM) has elected Stephen Klar as a director of the Company.

Mr. Klar was formerly the President and a Managing Partner of Wellington Management Group LLP.

CEO Liam Caffrey said, "We are delighted to welcome Steve to our Board of Directors. His extensive experience in the financial services industry will greatly benefit our owners."

About White Mountains

White Mountains is a Bermuda-domiciled financial services holding company traded on the New York Stock Exchange and the Bermuda Stock Exchange under the symbol “WTM” and “WTM.BH”, respectively.

XML

Document and Entity Information	Aug. 26, 2026
Cover [Abstract]
Document Type	8-K
Document Period End Date	Aug. 26, 2026
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP, LTD.
Entity Incorporation, State or Country Code	D0
Entity File Number	1-8993
Entity Tax Identification Number	94-2708455
Entity Address, Address Line One	23 South Main Street, Suite 3B
Entity Address, City or Town	Hanover
Entity Address, State or Province	NH
Entity Address, Postal Zip Code	03755
City Area Code	603
Local Phone Number	640-2200
Written Communications	false
Soliciting Material	false
Pre-commencement Tender Offer	false
Pre-commencement Issuer Tender Offer	false
Title of 12(b) Security	Common Shares, par value $1.00 per share
Trading Symbol	WTM
Security Exchange Name	NYSE
Entity Emerging Growth Company	false
Entity Central Index Key	0000776867
Amendment Flag	false