Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 28, 2025	Dec. 29, 2024
Income Statement [Abstract]
Revenue	$ 1,210,663	$ 1,420,561
Subcontractor costs	(173,487)	(223,231)
Other costs of revenue	(816,805)	(975,853)
Gross profit	220,371	221,477
Selling, general and administrative expenses	(86,824)	(84,317)
Legal contingency costs	0	(115,000)
Contingent consideration - fair value adjustments	7,447	366
Income from operations	140,994	22,526
Interest expense, net	(7,128)	(7,218)
Other non-operating income	7,710	0
Income before income tax expense	141,576	15,308
Income tax expense	(36,354)	(14,530)
Net income	105,222	778
Net income attributable to noncontrolling interests	(194)	(31)
Net income attributable to Tetra Tech	105,028	747
Net income attributable to Tetra Tech	$ 105,028	$ 747
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.40	$ 0
Diluted (in dollars per share)	$ 0.40	$ 0
Weighted-average common shares outstanding:
Basic (in shares)	260,809	267,854
Diluted (in shares)	262,731	271,886