Earnings per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 27, 2020	Dec. 29, 2019
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 52,436	$ 47,310
Weighted-average common shares outstanding – basic (in shares)	53,927	54,560
Effect of dilutive stock options and unvested restricted stock (in shares)	710	878
Weighted-average common stock outstanding – diluted (in shares)	54,637	55,438
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.97	$ 0.87
Diluted (in dollars per share)	$ 0.96	$ 0.85