Exhibit 32.1

CERTIFICATION OF PERIODIC REPORT PURSUANT TO

18 U.S.C. SECTION 1350,

AS ADOPTED PURSUANT TO SECTION 906

OF THE SARBANES-OXLEY ACT OF 2002

I, Juan Carlos Bueno, Chief Executive Officer of Mercer International Inc. (the “Company”), certify pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002 , that, to my knowledge:

(1)
the annual report on Form 10-K of the Company for the year ended December 31, 2025 (the “Report”) fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934 ; and
(2)
the information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

Dated: February 12, 2026

/s/ JUAN CARLOS BUENO
Juan Carlos Bueno
Chief Executive Officer

A signed original of this written statement required by Section 906 of the Sarbanes-Oxley Act of 2002 has been provided to Mercer International Inc. and will be retained by Mercer International Inc. and furnished to the Securities and Exchange Commission or its staff upon request.

This certification accompanies the Report pursuant to Section 906 of the Sarbanes-Oxley Act of 2002 and shall not, except to the extent required by the Sarbanes-Oxley Act of 2002, be deemed filed by the Company for purposes of Section 18 of the Securities Exchange Act of 1934 , as amended.