Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 29, 2024	Oct. 01, 2023	Oct. 02, 2022
Statement of Comprehensive Income [Abstract]
Net income	$ 333,443	$ 273,452	$ 263,164
Other comprehensive income (loss), net of tax
Foreign currency translation adjustments, net of tax	115,120	12,622	(94,933)
(Loss) gain on cash flow hedge valuations, net of tax	0	(2,412)	11,806
Net pension adjustments	1,300	2,638	0
Other comprehensive income (loss), net of tax	116,420	12,848	(83,127)
Comprehensive income, net of tax	449,863	286,300	180,037
Less: comprehensive income attributable to noncontrolling interests, net of tax	61	31	28
Comprehensive income attributable to Tetra Tech, net of tax	$ 449,802	$ 286,269	$ 180,009