Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 27, 2020	Sep. 29, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 173,890	$ 158,761	$ 136,957
Other comprehensive income, net of tax
Foreign currency translation adjustments, net of tax	3,436	(21,109)	(29,656)
(Loss) gain on cash flow hedge valuations, net of tax	(4,638)
(Loss) gain on cash flow hedge valuations, net of tax		(12,125)	806
Other comprehensive loss attributable to Tetra Tech, net of tax	(1,202)	(33,234)	(28,850)
Other comprehensive income (loss) attributable to noncontrolling interests, net of tax	(1)	243	(64)
Comprehensive income, net of tax	172,687	125,770	108,043
Comprehensive income attributable to Tetra Tech, net of tax	172,657	125,434	108,033
Comprehensive income attributable to noncontrolling interests, net of tax	$ 30	$ 336	$ 10