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 Great Lakes Dredge & Dock Corporation (the “Company”) on Form 10-Q for the period ended June 30, 2024, as filed with the Securities and Exchange Commission on the date hereof (the “Report”), I, Lasse J. Petterson, President and Chief Executive Officer of 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:

(1)The Report fully complies with the requirements of Section 13(a) or Section 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.

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 Great Lakes Dredge & Dock Corporation for purposes of Section 18 of the Securities Exchange Act of 1934, as amended.

/s/ LASSE J. PETTERSON

Lasse J. Petterson

President and Chief Executive Officer

Date: August 6, 2024

A signed original of this written statement required by Section 906, or other document authenticating, acknowledging, or otherwise adopting the signature that appears in typed form within the electronic version of this written statement required by Section 906, has been provided to Great Lakes Dredge & Dock Corporation and will be retained by Great Lakes Dredge & Dock Corporation and furnished to the Securities and Exchange Commission or its staff upon request.