Earnings per Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 30, 2025	Mar. 31, 2024	Mar. 30, 2025	Mar. 31, 2024
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 5,388	$ 76,446	$ 6,135	$ 151,418
Weighted-average common shares outstanding – basic (in shares)	265,728	267,420	266,819	267,095
Effect of dilutive stock options and unvested restricted stock (in shares)	1,711	1,955	1,936	2,030
Shares issuable assuming conversion of convertible notes (in shares)	0	0	936	0
Weighted-average common stock outstanding – diluted (in shares)	267,439	269,375	269,691	269,125
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.02	$ 0.29	$ 0.02	$ 0.57
Diluted (in dollars per share)	$ 0.02	$ 0.28	$ 0.02	$ 0.56