Earnings per Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 29, 2024	Dec. 31, 2023
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech, basic	$ 747	$ 74,972
Net income attributable to Tetra Tech, diluted	$ 747	$ 74,972
Weighted-average common shares outstanding – basic (in shares)	267,854	266,585
Effect of dilutive stock options and unvested restricted stock (in shares)	2,160	2,105
Shares issuable assuming conversion of convertible notes (in shares)	1,872	0
Weighted-average common stock outstanding – diluted (in shares)	271,886	268,690
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0	$ 0.28
Diluted (in dollars per share)	$ 0	$ 0.28