Earnings per Share - Schedule of Number of Weighted-Average Shares Used to Compute Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 28, 2025	Jun. 29, 2025	Mar. 30, 2025	Dec. 29, 2024	Sep. 29, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 28, 2025	Sep. 29, 2024	Oct. 01, 2023
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 127,745	$ 113,844	$ 5,388	$ 747	$ 96,154	$ 85,810	$ 76,446	$ 74,972	$ 247,724	$ 333,382	$ 273,420
Net income attributable to Tetra Tech	$ 127,745	$ 113,844	$ 5,388	$ 747	$ 96,154	$ 85,810	$ 76,446	$ 74,972	$ 247,724	$ 333,382	$ 273,420
Weighted-average common shares outstanding – basic (in shares)	262,184	263,026	265,728	267,854	267,687	267,575	267,420	266,585	264,713	267,364	266,015
Effect of diluted stock options and unvested restricted stock (in shares)									1,942	2,125	2,170
Shares issuable assuming conversion of convertible notes (in shares)									468	553	0
Weighted-average common stock outstanding – diluted (in shares)	264,247	264,855	267,439	271,886	271,656	270,260	269,375	268,690	267,123	270,042	268,185
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.49	$ 0.43	$ 0.02	$ 0	$ 0.36	$ 0.32	$ 0.29	$ 0.28	$ 0.94	$ 1.25	$ 1.03
Diluted (in dollars per share)	$ 0.48	$ 0.43	$ 0.02	$ 0	$ 0.35	$ 0.32	$ 0.28	$ 0.28	$ 0.93	$ 1.23	$ 1.02