September 2026 Business Tax Due Dates

September brings a wave of critical tax deadlines for businesses. Whether your company filed an extension earlier in the year or you are managing quarterly corporate estimated payments and employer payroll deposits, it is crucial to stay ahead of these requirements. Review the key September 2026 due dates below to help keep your business filings and payments on track.

Alarm clock representing tax due dates

September 15: S Corporations

If you requested an automatic six-month extension, you must file your 2025 calendar year income tax return (Form 1120-S) and pay any remaining 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. If applicable to your business, you must also furnish Schedule K-3 (Form 1120-S) or a substitute Schedule K-3.

September 15: Corporations

Calendar year corporations must deposit the third installment of their estimated income tax for the 2026 tax year by this date.

September 15: Partnerships

Partnerships that timely requested an automatic six-month extension must file their 2025 calendar year return (Form 1065) by September 15. In addition, you must provide each partner with a copy of their Schedule K-1 (Form 1065) or a substitute Schedule K-1, and if applicable, Schedule K-3 (Form 1065) or a substitute Schedule K-3.

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

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

September 30: Fiduciaries of Estates and Trusts

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

Weekends and Holidays

If any tax due date falls on a Saturday, Sunday, or a legal holiday, the deadline is automatically extended until the next business day that is not a legal holiday.

Disaster Area Extensions

When a specific geographical area is designated as a disaster area, tax filing and payment due dates are generally extended. To find more information on whether your area has been designated as a disaster area and to check for specific filing extension dates, visit the following official websites:

Maintain Your Business Compliance

Managing extended deadlines, payroll deposits, and quarterly estimates requires careful attention to detail. If you need assistance preparing your extended returns or calculating your remaining tax liabilities, please contact our office to schedule a consultation.

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