Earnings per Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jul. 02, 2023	Jul. 03, 2022	Jul. 02, 2023	Jul. 03, 2022
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech, basic	$ 60,235	$ 58,650	$ 219,771	$ 180,179
Net income attributable to Tetra Tech, diluted	$ 60,235	$ 58,650	$ 219,771	$ 180,179
Weighted-average common shares outstanding – basic (in shares)	53,231	53,507	53,188	53,777
Effect of dilutive stock options and unvested restricted stock (in shares)	422	499	427	551
Weighted-average common stock outstanding – diluted (in shares)	53,653	54,006	53,615	54,328
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 1.13	$ 1.10	$ 4.13	$ 3.35
Diluted (in dollars per share)	$ 1.12	$ 1.09	$ 4.10	$ 3.32