Mercer International (MERC) 10-Q 2018 Q1 Quarterly report

Exhibit 32.1

CERTIFICATION OF PERIODIC REPORT

I, David M. Gandossi, 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 theSarbanes-Oxley Actof 2002, that, to my knowledge:

(1) the Quarterly Report on Form10-Q of the Company for the quarterly period ended March 31, 2018 (the “Report”) fully complies with the requirements of Section 13(a) or 15(d) of theSecurities Exchange Actof 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: May 3, 2018

/s/ David M. Gandossi

David M. Gandossi

Chief Executive Officer

A signed original of this written statement required by Section 906 of theSarbanes-Oxley Act of 2002has 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 theSarbanes-Oxley Act of 2002 and shall not, except to the extent required by theSarbanes-Oxley Act of 2002, be deemed filed by the Company for purposes of Section 18 of theSecurities Exchange Act of 1934, as amended.