Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 28, 2025	Sep. 29, 2024	Oct. 01, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 247,949	$ 333,443	$ 273,452
Other comprehensive income (loss), net of tax
Foreign currency translation adjustments, net of tax	(17,165)	115,120	12,622
Loss on cash flow hedge valuations, net of tax	0	0	(2,412)
Net pension adjustments	263	1,300	2,638
Other comprehensive income (loss), net of tax	(16,902)	116,420	12,848
Comprehensive income, net of tax	231,047	449,863	286,300
Less: comprehensive income attributable to noncontrolling interests, net of tax	225	61	31
Comprehensive income attributable to Tetra Tech, net of tax	$ 230,822	$ 449,802	$ 286,269