Earnings per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 29, 2019	Dec. 30, 2018
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 47,310	$ 41,997
Weighted-average common shares outstanding – basic (in shares)	54,560	55,390
Effect of dilutive stock options and unvested restricted stock (in shares)	878	976
Weighted-average common stock outstanding – diluted (in shares)	55,438	56,366
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.87	$ 0.76
Diluted (in dollars per share)	$ 0.85	$ 0.75