Document and Entity Information - shares	3 Months Ended
	Dec. 27, 2015	Jan. 25, 2016
Document and Entity Information
Entity Registrant Name	TETRA TECH INC
Entity Central Index Key	0000831641
Document Type	10-Q
Document Period End Date	Dec. 27, 2015
Amendment Flag	false
Current Fiscal Year End Date	--10-02
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		58,624,247
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q1