Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Dec. 29, 2019	Dec. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 47,317	$ 42,032
Other comprehensive income, net of tax
Foreign currency translation adjustment, net of tax	13,899	(23,334)
Gain (loss) on cash flow hedge valuations, net of tax	1,670
Gain (loss) on cash flow hedge valuations, net of tax		(4,009)
Other comprehensive income (loss) attributable to Tetra Tech, net of tax	15,569	(27,343)
Other comprehensive income attributable to noncontrolling interests, net of tax	2	237
Comprehensive income attributable to Tetra Tech, net of tax	62,879	14,654
Comprehensive income attributable to noncontrolling interests, net of tax	9	272
Comprehensive income, net of tax	$ 62,888	$ 14,926