Document And Entity Information - USD ($)	12 Months Ended
	Sep. 29, 2018	Oct. 25, 2018	Mar. 31, 2018
Document Information [Line Items]
Entity Registrant Name	INSTEEL INDUSTRIES INC
Entity Central Index Key	0000764401
Trading Symbol	iiin
Current Fiscal Year End Date	--09-29
Entity Filer Category	Accelerated Filer
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding (in shares)		19,222,479
Entity Public Float			$ 437,197,613
Document Type	10-K
Document Period End Date	Sep. 29, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Shell Company	false