Earnings per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 28, 2021	Mar. 29, 2020	Mar. 28, 2021	Mar. 29, 2020
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 45,517	$ 36,397	$ 97,953	$ 83,707
Weighted-average common shares outstanding – basic (in shares)	54,187	54,699	54,085	54,541
Effect of dilutive stock options and unvested restricted stock (in shares)	549	764	630	839
Weighted-average common stock outstanding – diluted (in shares)	54,736	55,463	54,715	55,380
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.84	$ 0.67	$ 1.81	$ 1.53
Diluted (in dollars per share)	$ 0.83	$ 0.66	$ 1.79	$ 1.51