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. 03, 2021	Jun. 27, 2021	Mar. 28, 2021	Dec. 27, 2020	Sep. 27, 2020	Jun. 28, 2020	Mar. 29, 2020	Dec. 29, 2019	Oct. 03, 2021	Sep. 27, 2020	Sep. 29, 2019
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 892,012	$ 801,633	$ 754,764	$ 765,104	$ 753,364	$ 709,771	$ 734,133	$ 797,623	$ 3,213,513	$ 2,994,891	$ 3,107,348
Income from operations	81,836	69,807	60,807	66,252	66,735	63,525	47,530	63,302	278,701	241,091	188,762
Net income attributable to Tetra Tech	$ 82,954	$ 51,903	$ 45,517	$ 52,436	$ 44,654	$ 45,497	$ 36,397	$ 47,310	$ 232,810	$ 173,859	$ 158,668
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 1.54	$ 0.96	$ 0.84	$ 0.97	$ 0.83	$ 0.84	$ 0.67	$ 0.87	$ 4.31	$ 3.21	$ 2.89
Diluted (in dollars per share)	$ 1.52	$ 0.95	$ 0.83	$ 0.96	$ 0.82	$ 0.83	$ 0.66	$ 0.85	$ 4.26	$ 3.16	$ 2.84
Weighted-average common shares outstanding:
Basic (in shares)	54,019	54,117	54,187	53,927	53,841	53,985	54,699	54,560	54,078	54,235	54,986
Diluted (in shares)	54,597	54,666	54,736	54,637	54,603	54,692	55,463	55,438	54,675	55,022	55,936