UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported):

March 26, 2026

TETRA TECH, INC.

(Exact name of registrant as specified in its charter)

3475 East Foothill Boulevard, Pasadena, California 91107

(Address of principal executive office, including zip code)

(626) 351-4664

(Registrant’s telephone number, including area code)

Not Applicable

(Former name or former address, if changed since last report)

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:

Securities registered pursuant to Section 12(b) of the Act:

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 ¨

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. ¨

On March 26, 2026, Tetra Tech, Inc. (the “Company”) announced that Eric W. Thornburg has been appointed to the Board of Directors of the Company (the “Board”), effective March 30, 2026, to serve until the next Annual Meeting of Stockholders or until his successor has been duly chosen and qualified.  The Board will consist of eight members, including Mr. Thornburg, at that time.  Mr. Thornburg will serve as a member of the Compensation Committee and Nominating and Corporate Governance Committee. 

Mr. Thornburg, 65, most recently served as the Chief Executive Officer of H20 America (previously SJW Group) from 2017 to 2025 and Chair of the Board of Directors from 2018 to 2026. Prior to joining H20 America, he served as Chief Executive Officer of Connecticut Water Service from 2006 to 2017 and Chair of the Board of Directors from 2007 to 2017. Mr. Thornburg previously was Vice President of American Water Works Corporation from 2003 to 2006 and President of Missouri American Water from 2001 to 2003. He holds a B.A. degree in Biology and Business from Cornell University and an M.B.A. from Indiana Wesleyan University.

Mr. Thornburg will receive compensation for his service as a non-employee director in accordance with the Company’s non-employee director compensation program. Under this program, he will receive (1) an annual retainer of $110,000; (2) $2,000 for each meeting attended, when the number of Board meetings during the year has exceeded eight; and (3) an annual long-term equity award with a grant date fair value of $175,000, consisting sixty percent of performance stock units and forty percent of restricted stock units, each pro-rated for the portion of time that he is expected to serve as a director of the Company before the next annual award.

On March 26, 2026, the Company issued a press release announcing the matters described in Item 5.02. A copy of the press release is attached as Exhibit 99.1 and is incorporated by reference herein.

SIGNATURES

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.