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Cover - USD ($)
$ in Billions
 12 Months Ended
Oct. 01, 2023
Nov. 08, 2023
Apr. 02, 2023
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Oct. 01, 2023    
Current Fiscal Year End Date --10-01    
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 Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
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    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 7.7
Entity Common Stock, Shares Outstanding   53,247,668  
Documents Incorporated by Reference
DOCUMENT INCORPORATED BY REFERENCE
Portions of registrant's Proxy Statement for its 2024 Annual Meeting of Stockholders are incorporated by reference in Part III of this report where indicated.
    
Entity Central Index Key 0000831641    
Document Fiscal Year Focus 2023    
Document Fiscal Period Focus FY    
Amendment Flag false