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. 02, 2022	Jul. 03, 2022	Apr. 03, 2022	Jan. 02, 2022	Oct. 03, 2021	Jun. 27, 2021	Mar. 28, 2021	Dec. 27, 2020	Oct. 02, 2022	Oct. 03, 2021	Sep. 27, 2020
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 902,562	$ 890,231	$ 852,744	$ 858,510	$ 892,012	$ 801,633	$ 754,764	$ 765,104	$ 3,504,048	$ 3,213,513	$ 2,994,891
Income from operations	94,802	83,905	74,520	87,220	81,836	69,807	60,807	66,252	340,446	278,701	241,091
Net income attributable to Tetra Tech	82,947	58,650	53,040	68,489	82,954	51,903	45,517	52,436	263,125	232,810	173,859
Net income attributable to Tetra Tech	$ 82,947	$ 58,650	$ 53,040	$ 68,489	$ 82,954	$ 51,903	$ 45,517	$ 52,436	$ 263,125	$ 232,810	$ 173,859
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 1.56	$ 1.10	$ 0.99	$ 1.27	$ 1.54	$ 0.96	$ 0.84	$ 0.97	$ 4.91	$ 4.31	$ 3.21
Diluted (in dollars per share)	$ 1.55	$ 1.09	$ 0.98	$ 1.25	$ 1.52	$ 0.95	$ 0.83	$ 0.96	$ 4.86	$ 4.26	$ 3.16
Weighted-average common shares outstanding:
Basic (in shares)	53,148	53,507	53,834	53,937	54,019	54,117	54,187	53,927	53,620	54,078	54,235
Diluted (in shares)	53,667	54,006	54,346	54,577	54,597	54,666	54,736	54,637	54,163	54,675	55,022