Cover - shares	6 Months Ended
	Mar. 29, 2020	Apr. 22, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 29, 2020
Document Transition Report	false
Entity File Number	0-19655
Entity Registrant Name	TETRA TECH, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	95-4148514
Entity Address, Address Line One	3475 East Foothill Boulevard
Entity Address, City or Town	Pasadena
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	91107
City Area Code	626
Local Phone Number	351-4664
Title of 12(b) Security	Common Stock, $0.01 par value
Trading Symbol	TTEK
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		54,026,070
Entity Central Index Key	0000831641
Amendment Flag	false
Current Fiscal Year End Date	--09-27
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2