Earnings per Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jul. 03, 2022	Jun. 27, 2021	Jul. 03, 2022	Jun. 27, 2021
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech, basic	$ 58,650	$ 51,903	$ 180,179	$ 149,856
Net income attributable to Tetra Tech, diluted	$ 58,650	$ 51,903	$ 180,179	$ 149,856
Weighted-average common shares outstanding – basic (in shares)	53,507	54,117	53,777	54,095
Effect of dilutive stock options and unvested restricted stock (in shares)	499	549	551	603
Weighted-average common stock outstanding – diluted (in shares)	54,006	54,666	54,328	54,698
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 1.10	$ 0.96	$ 3.35	$ 2.77
Diluted (in dollars per share)	$ 1.09	$ 0.95	$ 3.32	$ 2.74