0001315257False00013152572025-11-052025-11-05

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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: (see General Instruction A.2. below)

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 o

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 On November 5, 2025, the Board of Directors (the “Board”) of Koppers Holdings Inc. (the “Company”) increased the size of the Board from eight to nine directors and elected Laura Posadas as a director of the Company. The committees on which Ms. Posadas will serve have not been determined. Ms. Posadas will receive compensation for her services as a non-employee director in accordance with the Company’s standard compensatory arrangement described in the Company’s Definitive Proxy Statement for its 2025 Annual Meeting under the caption “Director Compensation” filed pursuant to Regulation 14A with the Securities and Exchange Commission on March 28, 2025.

Since October 2024, Ms. Posadas has served as Chief Executive Officer of Canlak Coatings Inc., a formulator and manufacturer of high-quality wood coating systems. Ms. Posadas served as Chief Commercial Officer of Matrix Adhesives Group, a provider of advanced adhesive and sealant solutions, from January 2023 to October 2024 and as Regional President, Americas of Castrol for BP p.l.c., the global integrated energy company, from December 2019 to December 2022.

The Board has determined that Ms. Posadas is an independent director under applicable New York Stock Exchange listing requirements and the Board’s own independence standards, which are included in the Company’s Corporate Governance Guidelines. Ms. Posadas will stand for election by the Company’s shareholders at the Company’s 2026 Annual Meeting.

There are no transactions between Ms. Posadas and the Company that would be reportable under Item 404(a) of Regulation S-K, and there is no arrangement or understanding with any person pursuant to which Ms. Posadas was selected as a director.

The Company issued a press release on November 5, 2025 regarding the election of Ms. Posadas as a director. A copy of the press release is furnished herewith as Exhibit 99.1.

(d)Exhibits.

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 hereunto duly authorized.