Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 29, 2025	Jun. 30, 2024	Jun. 29, 2025	Jun. 30, 2024
Statement of Comprehensive Income [Abstract]
Net income	$ 113,883	$ 85,824	$ 120,073	$ 237,263
Other comprehensive income, net of tax
Foreign currency translation adjustment, net of tax	88,063	1,854	13,791	40,616
Net pension adjustments	0	3	(33)	(10)
Other comprehensive income, net of tax	88,063	1,857	13,758	40,606
Comprehensive income, net of tax	201,946	87,681	133,831	277,869
Less: Comprehensive income attributable to noncontrolling interests, net of tax	39	14	94	35
Comprehensive income attributable to Tetra Tech, net of tax	$ 201,907	$ 87,667	$ 133,737	$ 277,834