Document and Entity Information - shares	3 Months Ended	9 Months Ended
	Jul. 01, 2018	Jul. 01, 2018	Jul. 30, 2018
Document and Entity Information
Entity Registrant Name		TETRA TECH INC
Entity Central Index Key		0000831641
Document Type		10-Q
Document Period End Date		Jul. 01, 2018
Amendment Flag		false
Current Fiscal Year End Date		--09-30
Entity Current Reporting Status		Yes
Entity Filer Category		Large Accelerated Filer
Entity Common Stock, Shares Outstanding			55,333,840
Document Fiscal Year Focus		2018
Document Fiscal Period Focus	Q3