WHITE MOUNTAINS INSURANCE GROUP LTD

(Exact name of registrant as specified in its charter)

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:

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)