v3.25.4
Document and Entity Information - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2025
Jan. 22, 2026
Mar. 31, 2025
Cover [Abstract]      
Document Type 10-K/A    
Amendment Flag true    
Document Period End Date Sep. 30, 2025    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Entity Registrant Name PENNANTPARK INVESTMENT CORPORATION    
Entity Central Index Key 0001383414    
Current Fiscal Year End Date --09-30    
Document Annual Report true    
Document Transition Report false    
Entity File Number 814-00736    
Entity Tax Identification Number 20-8250744    
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    
Entity Shell Company false    
ICFR Auditor Attestation Flag true    
Entity Incorporation, State or Country Code MD    
Entity Address, Address Line One 1691 Michigan Avenue    
Entity Address, City or Town Miami Beach    
Entity Address, State or Province FL    
Entity Address, Postal Zip Code 33319    
City Area Code 786    
Local Phone Number 297-9500    
Title of 12(b) Security Common Stock, par value $0.001 per share    
Document Financial Statement Error Correction [Flag] false    
Trading Symbol PNNT    
Security Exchange Name NYSE    
Entity Public Float     $ 443.6
Entity Common Stock, Shares Outstanding   65,296,094  
Auditor Name RSM US LLP    
Auditor Firm ID 49    
Auditor Location New York, New York    
Documents Incorporated by Reference Documents Incorporated by Reference: Portions of the Registrant’s Proxy Statement relating to the Registrant’s 2026 Annual Meeting of Stockholders to be filed not later than 120 days after the end of the fiscal year covered by this Annual Report on Form 10-K are incorporated by reference into Part III of this Report.    
Amendment Description PennantPark Investment Corporation, a Maryland corporation, or together with its subsidiaries, where applicable, or the Company, which may also be referred to as “we,” “us” or “our,” is filing this Amendment No. 1, or this Amendment, to our Annual Report on Form 10-K for the fiscal year ended September 30, 2025, or the Form 10-K, which was initially filed with the Securities and Exchange Commission, or the SEC, on November 24, 2025. We are filing this Amendment to clarify that the financial information disclosed in Note 13 for JF Intermediate, LLC and AKW Holdings Limited was audited as part of RSM US LLP’s audit of the consolidated statements of assets and liabilities of the Company and its subsidiaries, including the consolidated schedules of investments, as of September 30, 2025 and 2024, and the related consolidated statements of operations, changes in net assets and cash flows for each of the three years in the period ended September 30, 2025, and the related notes to the consolidated financial statements. This Amendment also includes the filing of new Exhibits 31.1, 31.2, 32.1 and 32.2, certifications of our Chief Executive Officer and Chief Financial Officer, pursuant to Rule 13a-14(a) and (b) of the Securities Exchange Act of 1934, as amended. Except as described above, no other changes have been made to the Form 10-K. This Amendment does not reflect subsequent events that may have occurred after the original filing date of the Form 10-K or modify or update in any way disclosures made in the Form 10-K, except as required to reflect the revisions discussed above. Among other things, forward-looking statements made in the Form 10-K have not been revised to reflect events that occurred or facts that became known to us after filing of the Form 10-K, and such forward-looking statements should be read in their historical context. Furthermore, this Amendment should be read in conjunction with the Form 10-K and with our subsequent filings with the SEC.