Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 29, 2025	Jun. 30, 2024	Jun. 29, 2025	Jun. 30, 2024
Revenue	$ 1,369,816	$ 1,344,323	$ 4,112,490	$ 3,824,205
Gross profit	251,539	223,172	687,197	621,165
Selling, general and administrative expenses	(86,611)	(94,042)	(255,022)	(263,293)
Legal contingency costs	0	0	(115,000)	0
Contingent consideration – fair value adjustments	58	(500)	2,355	(477)
Goodwill impairment	0	0	(92,416)	0
Income from operations	164,986	128,630	227,114	357,395
Interest expense, net	(8,288)	(9,912)	(23,996)	(29,374)
Income before income tax expense	156,698	118,718	203,118	328,021
Income tax expense	(42,815)	(32,894)	(83,045)	(90,758)
Net income	113,883	85,824	120,073	237,263
Net income attributable to noncontrolling interests	(39)	(14)	(94)	(35)
Net income attributable to Tetra Tech	$ 113,844	$ 85,810	$ 119,979	$ 237,228
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
Weighted-average common shares outstanding:
Basic (in shares)	263,026	267,575	265,589	267,255
Diluted (in shares)	264,855	270,260	268,113	269,503
Subcontractor costs
Cost of revenue	$ (216,800)	$ (234,742)	$ (658,439)	$ (646,828)
Other costs of revenue
Cost of revenue	$ (901,477)	$ (886,409)	$ (2,766,854)	$ (2,556,212)