Earnings per Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Apr. 02, 2023	Mar. 31, 2024	Apr. 02, 2023
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech, basic	$ 76,446	$ 42,830	$ 151,418	$ 159,536
Net income attributable to Tetra Tech, diluted	$ 76,446	$ 42,830	$ 151,418	$ 159,536
Weighted-average common shares outstanding – basic (in shares)	53,484	53,227	53,419	53,165
Effect of dilutive stock options and unvested restricted stock (in shares)	391	400	406	430
Weighted-average common stock outstanding – diluted (in shares)	53,875	53,627	53,825	53,595
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 1.43	$ 0.80	$ 2.83	$ 3.00
Diluted (in dollars per share)	$ 1.42	$ 0.80	$ 2.81	$ 2.98