Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jul. 02, 2023	Jun. 30, 2024	Jul. 02, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 85,824	$ 60,238	$ 237,263	$ 219,794
Other comprehensive income, net of tax
Foreign currency translation adjustment, net of tax	1,854	44,500	40,616	69,453
Loss on cash flow hedge valuations, net of tax	0	(1,044)	0	(2,029)
Net pension adjustments	3	0	(10)	2,794
Other comprehensive income, net of tax	1,857	43,456	40,606	70,218
Comprehensive income, net of tax	87,681	103,694	277,869	290,012
Less: Comprehensive income attributable to noncontrolling interests, net of tax	14	4	35	23
Comprehensive income attributable to Tetra Tech, net of tax	$ 87,667	$ 103,690	$ 277,834	$ 289,989