Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Oct. 02, 2016	Nov. 07, 2016	Mar. 24, 2016
Document and Entity Information
Entity Registrant Name	TETRA TECH INC
Entity Central Index Key	0000831641
Document Type	10-K
Document Period End Date	Oct. 02, 2016
Amendment Flag	false
Current Fiscal Year End Date	--10-02
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 1.4
Entity Common Stock, Shares Outstanding		57,060,803
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	FY