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Document And Entity Information - USD ($)
 12 Months Ended
Sep. 30, 2023
Oct. 24, 2023
Apr. 01, 2023
Document Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Sep. 30, 2023    
Document Transition Report false    
Entity File Number 1-9929    
Entity Registrant Name INSTEEL INDUSTRIES INC.    
Entity Incorporation, State or Country Code NC    
Entity Tax Identification Number 56-0674867    
Entity Address, Address Line One 1373 Boggs Drive    
Entity Address, City or Town Mount Airy    
Entity Address, State or Province NC    
Entity Address, Postal Zip Code 27030    
City Area Code 336    
Local Phone Number 786-2141    
Title of 12(b) Security Common Stock (No Par Value)    
Trading Symbol IIIN    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Public Float     $ 424,911,914
Entity Common Stock, Shares Outstanding (in shares)   19,454,250  
Auditor Name Grant Thornton LLP    
Auditor Firm ID 248    
Auditor Location Charlotte, North Carolina    
Entity Central Index Key 0000764401    
Current Fiscal Year End Date --09-30    
Document Fiscal Year Focus 2023    
Document Fiscal Period Focus FY    
Amendment Flag false