Quarterly Financial Information – Unaudited - Summary of Quarterly Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Oct. 01, 2023	Jul. 02, 2023	Apr. 02, 2023	Jan. 01, 2023	Oct. 02, 2022	Jul. 03, 2022	Apr. 03, 2022	Jan. 02, 2022	Oct. 01, 2023	Oct. 02, 2022	Oct. 03, 2021
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 1,260,611	$ 1,208,947	$ 1,158,226	$ 894,766	$ 902,562	$ 890,231	$ 852,744	$ 858,510	$ 4,522,550	$ 3,504,048	$ 3,213,513
Income from operations	107,377	97,675	61,011	92,050	94,802	83,905	74,520	87,220	358,113	340,446	278,701
Net income attributable to Tetra Tech	53,649	60,235	42,830	116,706	82,947	58,650	53,040	68,489	273,420	263,125	232,810
Net income attributable to Tetra Tech	$ 53,649	$ 60,235	$ 42,830	$ 116,706	$ 82,947	$ 58,650	$ 53,040	$ 68,489	$ 273,420	$ 263,125	$ 232,810
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 1.01	$ 1.13	$ 0.80	$ 2.20	$ 1.56	$ 1.10	$ 0.99	$ 1.27	$ 5.14	$ 4.91	$ 4.31
Diluted (in dollars per share)	$ 1.00	$ 1.12	$ 0.80	$ 2.18	$ 1.55	$ 1.09	$ 0.98	$ 1.25	$ 5.10	$ 4.86	$ 4.26
Weighted-average common shares outstanding:
Basic (in shares)	53,247	53,231	53,227	53,069	53,148	53,507	53,834	53,937	53,203	53,620	54,078
Diluted (in shares)	53,702	53,653	53,627	53,529	53,667	54,006	54,346	54,577	53,637	54,163	54,675