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. 29, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Oct. 01, 2023	Jul. 02, 2023	Apr. 02, 2023	Jan. 01, 2023	Sep. 29, 2024	Oct. 01, 2023	Oct. 02, 2022
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 96,154	$ 85,810	$ 76,446	$ 74,972	$ 53,649	$ 60,235	$ 42,830	$ 116,706	$ 333,382	$ 273,420	$ 263,125
Net income attributable to Tetra Tech	$ 96,154	$ 85,810	$ 76,446	$ 74,972	$ 53,649	$ 60,235	$ 42,830	$ 116,706	$ 333,382	$ 273,420	$ 263,125
Weighted-average common shares outstanding – basic (in shares)	267,687	267,575	267,420	266,585	266,235	266,155	266,135	265,345	267,364	266,015	268,100
Effect of diluted stock options and unvested restricted stock (in shares)									2,125	2,170	2,715
Shares issuable assuming conversion of convertible notes (in shares)									553	0	0
Weighted-average common stock outstanding – diluted (in shares)	271,656	270,260	269,375	268,690	268,510	268,265	268,135	267,645	270,042	268,185	270,815
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.36	$ 0.32	$ 0.29	$ 0.28	$ 0.20	$ 0.23	$ 0.16	$ 0.44	$ 1.25	$ 1.03	$ 0.98
Diluted (in dollars per share)	$ 0.35	$ 0.32	$ 0.28	$ 0.28	$ 0.20	$ 0.22	$ 0.16	$ 0.44	$ 1.23	$ 1.02	$ 0.97