Earnings per Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Jan. 02, 2022	Dec. 27, 2020
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 68,489	$ 52,436
Weighted-average common shares outstanding – basic (in shares)	53,937	53,927
Effect of dilutive stock options and unvested restricted stock (in shares)	640	710
Weighted-average common stock outstanding – diluted (in shares)	54,577	54,637
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 1.27	$ 0.97
Diluted (in dollars per share)	$ 1.25	$ 0.96