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September 2026 Business Tax Deadlines

September brings several critical tax deadlines for business owners. Whether you lead a professional service firm, a medical practice, or a real estate agency, staying ahead of these dates is essential for minimizing stress and maintaining control over your financials. This month includes extended return deadlines for S corporations and partnerships, corporate estimated tax payments, and employer payroll tax deposits. Review the key due dates below to keep your filings and payments on schedule.

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September 15: S Corporations

If you requested an automatic 6-month extension for your S corporation, you must file your 2025 calendar year income tax return (Form 1120-S) and pay any tax due by September 15. You are also required to provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a substitute Schedule K-1, as well as Schedule K-3 (Form 1120-S) if applicable.

September 15: Corporations

For calendar year corporations, this date marks the deadline to deposit the third installment of estimated income tax for 2026.

September 15: Partnerships

If your partnership timely requested an automatic 6-month extension, you must file your 2025 calendar year return (Form 1065). Additionally, provide each partner with a copy of their Schedule K-1 (Form 1065) or a substitute, along with Schedule K-3 (Form 1065) if applicable.

September 15: Social Security, Medicare, and Withheld Income Tax

If you are an employer and the monthly deposit rules apply to your business, September 15 is the deadline to deposit Social Security, Medicare, and withheld income tax for August 2026. This is also the due date for the August 2026 nonpayroll withholding deposit if the monthly deposit rule applies.

September 30: Fiduciaries of Estates and Trusts

If you were granted a 5½-month extension, you must file a 2025 calendar year return (Form 1041) by September 30. If applicable, provide each beneficiary with a copy of their Schedule K-1 (Form 1041) or a substitute Schedule K-1.

Weekends, Holidays, and Disaster Area Extensions

Weekends and Holidays: If any due date happens to fall on a Saturday, Sunday, or legal holiday, the deadline is automatically extended to the next business day that is not a legal holiday.

Disaster Area Extensions: Due dates are also extended when a geographical area is designated as a disaster area. To find out if an area has received a disaster designation and to check filing extension dates, visit the following official websites:

Maintain Financial Control with Get Balanced CPA

Managing extended deadlines and quarterly estimated payments takes time away from running your business. At Get Balanced CPA, led by Sam Faulkner, our team integrates real-world experience with modern, tech-forward tools to bring clarity to your financials and help minimize your tax burden. If you need assistance navigating these September deadlines, reach out to our Cumming, GA office today to schedule a consultation.

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