Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Oct. 03, 2021	Sep. 27, 2020	Sep. 29, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 232,831	$ 173,890	$ 158,761
Other comprehensive income, net of tax
Foreign currency translation adjustments, net of tax	30,644	3,435	(20,866)
Gain (loss) on cash flow hedge valuations, net of tax	6,117	(4,638)
Gain (loss) on cash flow hedge valuations, net of tax			(12,125)
Other comprehensive income (loss), net of tax	36,761	(1,203)	(32,991)
Comprehensive income, net of tax	269,592	172,687	125,770
Comprehensive income attributable to noncontrolling interests, net of tax	24	30	336
Comprehensive income attributable to Tetra Tech, net of tax	$ 269,568	$ 172,657	$ 125,434