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Key Business Tax Deadlines for September 2025

As September 2025 approaches, business entities must be prepared for critical tax deadlines. Key filings include those for S corporations, partnerships, and corporations with extension-specific returns due, as well as essential estimated payments and tax deposits. Ensuring compliance with these dates is crucial to avoid penalties. Here's a comprehensive guide to what's due this September.

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September 15 - S Corporations
On this date, S corporations need to file their 2024 calendar year income tax return using Form 1120-S and settle any tax due. This schedule applies if an automatic 6-month extension was previously obtained. Each shareholder must receive a proper Schedule K-1 (Form 1120-S) or a substitute, plus a Schedule K-3 if applicable.

September 15 - Corporations

It’s time for calendar year corporations to deposit the third installment of estimated income tax for 2025, ensuring continuous compliance with federal tax obligations.

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September 15 - Partnerships

Partnerships must finalize their 2024 calendar year return utilizing Form 1065 if they have secured a 6-month extension. Partners should be presented with either their Schedule K-1 (Form 1065) or an equivalent, along with the necessary Schedule K-3 if applicable.

September 15 - Social Security, Medicare, and Withheld Income Tax
For employers adhering to monthly deposit guidelines, this date is also vital for depositing August 2025’s Social Security, Medicare, and withheld income taxes. Additionally, nonpayroll withholding deposits for August 2025 must be made if the monthly deposit rule is applicable.

Weekends & Holidays:

In instances where a due date coincides with a weekend or federal holiday, deadlines are automatically extended to the next business day.

Disaster Area Extensions:
In designated disaster areas, additional time to file is often granted. For updates on disaster declarations and extended filing dates, please visit:

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