Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Sep. 30, 2018	Nov. 01, 2018	Apr. 01, 2018
Document and Entity Information
Entity Registrant Name	TETRA TECH INC
Entity Central Index Key	0000831641
Current Fiscal Year End Date	--09-30
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Document Type	10-K
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Common Stock, Shares Outstanding		55,356,389
Entity Public Float			$ 2.6