Exhibit 32.1

CERTIFICATION PURSUANT TO

18 U.S.C. SECTION 1350,

AS ADOPTED PURSUANT TO

SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

In connection with the Quarterly Report of Koppers Holdings Inc. (the “Company”) on Form 10-Q for the quarter ending March 31, 2024 as filed with the Securities and Exchange Commission on the date hereof (the “Report”), each of the undersigned hereby certifies in his or her capacity as an officer of the Company, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that:

(1)The Report fully complies with the requirements of section 13(a) or 15(d) of the Securities Exchange Act of 1934, as amended; and

(2)The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

/s/ LEROY M. BALL	/S/ JIMMI SUE SMITH
Leroy M. Ball	Jimmi Sue Smith
President and Chief Executive Officer	Chief Financial Officer
May 3, 2024	May 3, 2024