Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 27, 2021	Jun. 28, 2020	Jun. 27, 2021	Jun. 28, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 51,924	$ 45,503	$ 149,900	$ 129,234
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	11,159	21,685	54,527	(11,428)
Gain (loss) on cash flow hedge valuations, net of tax	1,498	(30)	4,864	(5,726)
Other comprehensive income (loss), net of tax	12,657	21,655	59,391	(17,154)
Comprehensive income, net of tax	64,581	67,158	209,291	112,080
Comprehensive income attributable to noncontrolling interests, net of tax	23	2	50	23
Comprehensive income attributable to Tetra Tech, net of tax	$ 64,558	$ 67,156	$ 209,241	$ 112,057