Earnings per Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 29, 2025	Jun. 30, 2024	Jun. 29, 2025	Jun. 30, 2024
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 113,844	$ 85,810	$ 119,979	$ 237,228
Weighted-average common shares outstanding – basic (in shares)	263,026	267,575	265,589	267,255
Effect of dilutive stock options and unvested restricted stock (in shares)	1,829	2,165	1,900	2,073
Shares issuable assuming conversion of convertible notes (in shares)	0	520	624	175
Weighted-average common stock outstanding – diluted (in shares)	264,855	270,260	268,113	269,503
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.43	$ 0.32	$ 0.45	$ 0.89
Diluted (in dollars per share)	$ 0.43	$ 0.32	$ 0.45	$ 0.88