Earnings per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 27, 2021	Jun. 28, 2020	Jun. 27, 2021	Jun. 28, 2020
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 51,903	$ 45,497	$ 149,856	$ 129,205
Weighted-average common shares outstanding – basic (in shares)	54,117	53,985	54,095	54,366
Effect of dilutive stock options and unvested restricted stock (in shares)	549	707	603	795
Weighted-average common stock outstanding – diluted (in shares)	54,666	54,692	54,698	55,161
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.96	$ 0.84	$ 2.77	$ 2.38
Diluted (in dollars per share)	$ 0.95	$ 0.83	$ 2.74	$ 2.34