Quarterly Financial Information – Unaudited (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
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 1,330,100	$ 1,369,816	$ 1,322,113	$ 1,420,561	$ 1,374,473	$ 1,344,323	$ 1,251,616	$ 1,228,267	$ 5,442,590	$ 5,198,679	$ 4,522,550
Income from operations	181,304	164,986	39,603	22,526	143,343	128,630	117,683	111,081	408,419	500,737	358,113
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
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
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
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