Document and Entity Information - shares	3 Months Ended
	Jan. 01, 2017	Jan. 30, 2017
Document and Entity Information
Entity Registrant Name	TETRA TECH INC
Entity Central Index Key	0000831641
Document Type	10-Q
Document Period End Date	Jan. 01, 2017
Amendment Flag	false
Current Fiscal Year End Date	--10-01
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		57,317,053
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q1