Earnings per Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2023	Jan. 01, 2023
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech, basic	$ 74,972	$ 116,706
Net income attributable to Tetra Tech, diluted	$ 74,972	$ 116,706
Weighted-average common shares outstanding – basic (in shares)	53,317	53,069
Effect of dilutive stock options and unvested restricted stock (in shares)	421	460
Weighted-average common stock outstanding – diluted (in shares)	53,738	53,529
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 1.41	$ 2.20
Diluted (in dollars per share)	$ 1.40	$ 2.18