Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Dec. 27, 2020	Dec. 29, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 52,448	$ 47,317
Other comprehensive income, net of tax
Foreign currency translation adjustment, net of tax	32,393	13,901
Gain on cash flow hedge valuations, net of tax	1,476	1,670
Other comprehensive income, net of tax	33,869	15,571
Comprehensive income, net of tax	86,317	62,888
Comprehensive income attributable to noncontrolling interests, net of tax	14	9
Comprehensive income attributable to Tetra Tech, net of tax	$ 86,303	$ 62,879