Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 28, 2025	Sep. 29, 2024	Oct. 01, 2023
Income Statement [Abstract]
Revenue	$ 5,442,590	$ 5,198,679	$ 4,522,550
Costs of revenue	(825,230)	(876,817)	(771,461)
Other costs of revenue	(3,656,016)	(3,455,422)	(3,026,060)
Gross profit	961,344	866,440	725,029
Selling, general and administrative expenses	(357,737)	(356,024)	(305,107)
Legal contingency costs	(115,000)	0	0
Impairment of goodwill	(92,416)	0	0
Acquisition and integration expenses	0	(7,138)	(33,169)
Right-of-use operating lease asset impairment	0	0	(16,385)
Contingent consideration – fair value adjustments	12,228	(2,541)	(12,255)
Income from operations	408,419	500,737	358,113
Interest income	9,837	7,288	5,898
Interest expense	(40,639)	(44,559)	(52,435)
Other non-operating income	0	0	89,402
Income before income tax expense	377,617	463,466	400,978
Income tax expense	(129,668)	(130,023)	(127,526)
Net income	247,949	333,443	273,452
Net income attributable to noncontrolling interests	(225)	(61)	(32)
Net income attributable to Tetra Tech	247,724	333,382	273,420
Net income attributable to Tetra Tech	$ 247,724	$ 333,382	$ 273,420
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.94	$ 1.25	$ 1.03
Diluted (in dollars per share)	$ 0.93	$ 1.23	$ 1.02
Weighted-average common shares outstanding:
Basic (in shares)	264,713	267,364	266,015
Diluted (in shares)	267,123	270,042	268,185