Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Jan. 01, 2023	Jan. 02, 2022
Statement of Comprehensive Income [Abstract]
Net income	$ 116,715	$ 68,499
Other comprehensive income, net of tax
Foreign currency translation adjustment, net of tax	33,107	(686)
Gain (loss) on cash flow hedge valuations, net of tax	(89)	2,666
Other comprehensive income, net of tax	33,018	1,980
Comprehensive income, net of tax	149,733	70,479
Comprehensive income attributable to noncontrolling interests, net of tax	9	10
Comprehensive income attributable to Tetra Tech, net of tax	$ 149,724	$ 70,469