Earnings per Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 03, 2022	Mar. 28, 2021	Apr. 03, 2022	Mar. 28, 2021
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 53,040	$ 45,517	$ 121,529	$ 97,953
Weighted-average common shares outstanding – basic (in shares)	53,834	54,187	53,916	54,085
Effect of dilutive stock options and unvested restricted stock (in shares)	512	549	576	630
Weighted-average common stock outstanding – diluted (in shares)	54,346	54,736	54,492	54,715
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.99	$ 0.84	$ 2.25	$ 1.81
Diluted (in dollars per share)	$ 0.98	$ 0.83	$ 2.23	$ 1.79