Earnings per Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 02, 2023	Apr. 03, 2022	Apr. 02, 2023	Apr. 03, 2022
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech, basic	$ 42,830	$ 53,040	$ 159,536	$ 121,529
Net income attributable to Tetra Tech, diluted	$ 42,830	$ 53,040	$ 159,536	$ 121,529
Weighted-average common shares outstanding – basic (in shares)	53,227	53,834	53,165	53,916
Effect of dilutive stock options and unvested restricted stock (in shares)	400	512	430	576
Weighted-average common stock outstanding – diluted (in shares)	53,627	54,346	53,595	54,492
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.80	$ 0.99	$ 3.00	$ 2.25
Diluted (in dollars per share)	$ 0.80	$ 0.98	$ 2.98	$ 2.23