If your S Corporation filed for a tax extension earlier this year, September 15, 2025 is your final deadline to submit Form 1120-S. Missing this can result in steep penalties, disruption in shareholder reporting, and even risk to your S Corporation status. 


Filing Form 1120 for a New Corporation is often the first compliance step for businesses that are initially formed as C Corporations and later plan to elect S Corporation status.  


Why Filing Form 1120-S Matters 


S Corporations don’t pay federal income tax directly. Profits and losses are directly transferred to shareholders, who are responsible for reporting them on their tax returns. However, the S Corporation is still responsible for filing Form 1120-S and issuing each shareholder a timely Schedule K-1 to stay compliant. 

Filing late—even by a few days—can lead to IRS penalties of $235 per shareholder per month, up to 12 months. 


Key Deadlines 

  • Original Deadline: March 17, 2025 
  • Extended Deadline: September 15, 2025 (only applies if you filed Form 7004 before March 17) 
  • Note: The extension only grants you extra time to file your paperwork; it doesn’t change the deadline for payment. Shareholders were still required to pay any tax owed by the original deadline.  


What You’ll Need to File 


To complete your S Corp filing, make sure you have: 


  • Form 1120-S – main return 
  • Schedule K-1 – issued to each shareholder 
  • Form 7004 – if you requested an extension 
  • Form 2553 – If your business is choosing S Corporation status for the first time 

Depending on your situation, you might also need to attach schedules like B, L, M-1, and M-2. 


If you’re confused about Form 1065 vs Form 1120-S, you’re not alone. Both are used by pass-through entities, but they apply to different structures; Form 1065 is for partnerships, while Form 1120-S is for S Corporations. 


Federal vs. State Requirements 


Some states like California, New York, and New Jersey have separate filing or election requirements for S Corporations. Double-check both federal and state deadlines to stay compliant. 


IRS Trends to Watch in 2025 


The IRS is increasing scrutiny of S Corporations in 2025. Focus areas include: 

  • Failure to pay reasonable compensation to shareholder-employees 
  • Missing or late K-1 filings 
  • Repeated late Form 1120-S submissions 

More details are available on the IRS Compliance Campaigns page. 


Avoid These Common Mistakes 

  • Filing late after September 15 
  • Delaying or skipping K-1 distribution 
  • Assuming the extension covers tax payments 
  • Overlooking state requirements 
  • Misreporting shareholder salaries 

Best Practices to Stay Compliant 

  • Start preparations by early August 
  • Work with a qualified CPA 
  • Use secure e-filing for faster processing  
  • Issue K-1s well before the deadline 
  • Set calendar reminders to stay on track 


Final Note 


The S Corp filing deadline 2025 isn’t just about avoiding penalties—it’s about maintaining IRS compliance and giving your business a smooth financial year-end. 

Need help staying compliant? 


Book a free consultation with us today and ensure you're ready before the September 15 deadline.