Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 03, 2022	Mar. 28, 2021	Apr. 03, 2022	Mar. 28, 2021
Statement of Comprehensive Income [Abstract]
Net income	$ 53,049	$ 45,528	$ 121,548	$ 97,976
Other comprehensive income, net of tax
Foreign currency translation adjustment, net of tax	2,807	10,975	2,121	43,368
Gain on cash flow hedge valuations, net of tax	4,938	1,890	7,604	3,366
Other comprehensive income, net of tax	7,745	12,865	9,725	46,734
Comprehensive income, net of tax	60,794	58,393	131,273	144,710
Comprehensive income attributable to noncontrolling interests, net of tax	10	13	19	27
Comprehensive income attributable to Tetra Tech, net of tax	$ 60,784	$ 58,380	$ 131,254	$ 144,683