Earnings per Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jul. 02, 2023	Jun. 30, 2024	Jul. 02, 2023
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech, basic	$ 85,810	$ 60,235	$ 237,228	$ 219,771
Net income attributable to Tetra Tech, diluted	$ 85,810	$ 60,235	$ 237,228	$ 219,771
Weighted-average common shares outstanding – basic (in shares)	53,515	53,231	53,451	53,188
Effect of dilutive stock options and unvested restricted stock (in shares)	433	422	415	427
Shares issuable assuming conversion of convertible notes (in shares)	104	0	35	0
Weighted-average common stock outstanding – diluted (in shares)	54,052	53,653	53,901	53,615
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 1.60	$ 1.13	$ 4.44	$ 4.13
Diluted (in dollars per share)	$ 1.59	$ 1.12	$ 4.40	$ 4.10