Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Oct. 01, 2023	Oct. 02, 2022	Oct. 03, 2021
Statement of Comprehensive Income [Abstract]
Net income	$ 273,452	$ 263,164	$ 232,831
Other comprehensive income, net of tax
Foreign currency translation adjustments, net of tax	12,622	(94,933)	30,644
(Loss) gain on cash flow hedge valuations, net of tax	(2,412)	11,806	6,117
Net pension adjustments	2,638	0	0
Other comprehensive income (loss), net of tax	12,848	(83,127)	36,761
Comprehensive income, net of tax	286,300	180,037	269,592
Comprehensive income attributable to noncontrolling interests, net of tax	31	28	24
Comprehensive income attributable to Tetra Tech, net of tax	$ 286,269	$ 180,009	$ 269,568