Essential September 2025 Tax Deadlines for Business Owners

The tax season brings critical milestones, and September 2025 presents several key due dates for business filings. Among these are the deadlines for extended S Corporation and partnership returns, corporate estimated payments, and payroll tax deposits, all falling on September 15.

September 15 - S Corporations
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Businesses operating as S Corporations need to file their 2024 calendar year income tax return (Form 1120-S) by this date if they had previously received a 6-month automatic extension. It's essential to ensure that each shareholder receives their Schedule K-1 (Form 1120-S) or a substitute, and if required, their Schedule K-3 or its substitute.

September 15 - Corporations

Corporations adhering to a calendar year schedule must deposit their third installment of estimated income tax for 2025 by this date.

September 15 - Partnerships
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Partnerships that filed for a 6-month extension are required to submit their 2024 calendar year return (Form 1065) by September 15. Partners should receive their Schedule K-1 (Form 1065) or a substitute, and potentially a Schedule K-3 (Form 1065) or its substitute.

September 15 - Social Security, Medicare, and Withheld Income Tax
Employers following the monthly deposit rules must deposit Social Security, Medicare, and withheld income taxes for August 2025 by September 15. Similarly, if under the monthly deposit rule, the non-payroll withholding deposit for August 2025 is also due.

Considerations for Weekends & Holidays:
If a deadline falls on a weekend or legal holiday, the due date extends to the following business day.

Disaster Area Extensions:
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Special extensions apply to areas designated as disaster zones. For ongoing updates, check FEMA's disaster declarations and the IRS's tax relief updates.

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