Earnings per Share - Calculation (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
Earnings Per Share [Abstract]
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
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
Effect of diluted stock options and unvested restricted stock (in shares)									597	787	950
Weighted-average common stock outstanding – 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
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