Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Jan. 02, 2022	Dec. 27, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 68,499	$ 52,448
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment, net of tax	(686)	32,393
Gain on cash flow hedge valuations, net of tax	2,666	1,476
Other comprehensive income, net of tax	1,980	33,869
Comprehensive income, net of tax	70,479	86,317
Comprehensive income attributable to noncontrolling interests, net of tax	10	14
Comprehensive income attributable to Tetra Tech, net of tax	$ 70,469	$ 86,303