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Document And Entity Information - USD ($)
 12 Months Ended
Sep. 30, 2017
Oct. 26, 2017
Apr. 01, 2017
Document Information [Line Items]      
Entity Registrant Name INSTEEL INDUSTRIES INC    
Entity Central Index Key 0000764401    
Trading Symbol iiin    
Current Fiscal Year End Date --09-30    
Entity Filer Category Accelerated Filer    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Well-known Seasoned Issuer No    
Entity Common Stock, Shares Outstanding (in shares)   19,041,046  
Entity Public Float     $ 579,089,435
Document Type 10-K    
Document Period End Date Sep. 30, 2017    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Amendment Flag false