Earnings per Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Jan. 01, 2023	Jan. 02, 2022
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech, basic	$ 116,706	$ 68,489
Net income attributable to Tetra Tech, diluted	$ 116,706	$ 68,489
Weighted-average common shares outstanding – basic (in shares)	53,069	53,937
Effect of dilutive stock options and unvested restricted stock (in shares)	460	640
Weighted-average common stock outstanding – diluted (in shares)	53,529	54,577
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 2.20	$ 1.27
Diluted (in dollars per share)	$ 2.18	$ 1.25