Key September 2026 Tax Deadlines for Businesses

September introduces a series of crucial tax deadlines for businesses. Whether you are managing extended returns for S corporations and partnerships, remitting corporate estimated tax payments, or handling employer payroll tax deposits, staying organized is essential. Review the key September 2026 due dates below to ensure your business filings and payments remain on track.

Tax forms and documentation

September 15: S Corporation Extended Returns

If you requested an automatic six-month extension for your 2025 calendar year income tax return, your Form 1120-S is due on September 15. In addition to filing the return and paying any tax due, you must provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a substitute Schedule K-1. If applicable, you must also provide Schedule K-3 (Form 1120-S) or its substitute.

September 15: Corporate Estimated Tax Payments

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

September 15: Partnership Extended Returns

Similar to S corporations, partnerships that timely requested an automatic six-month extension must file their 2025 calendar year return (Form 1065) by September 15. Ensure every partner receives a copy of their Schedule K-1 (Form 1065) or a substitute, alongside Schedule K-3 (Form 1065) or its substitute if applicable.

September 15: Payroll and Nonpayroll Withholding Deposits

For employers subject to monthly deposit rules, September 15 is the deadline to deposit Social Security, Medicare, and withheld income tax for August 2026. This date also applies to the nonpayroll withholding deposit for August 2026 if your business falls under the monthly deposit rule.

September 30: Fiduciaries of Estates and Trusts

Fiduciaries operating on a calendar year must file Form 1041 for the 2025 tax year if they were granted a 5½-month extension. If applicable, you must also provide each beneficiary with a copy of their Schedule K-1 (Form 1041) or a substitute Schedule K-1.

Exceptions for Weekends, Holidays, and Disaster Areas

The IRS automatically extends due dates that fall on a Saturday, Sunday, or legal holiday to the next business day that is not a legal holiday.

Additionally, businesses located in designated disaster areas may qualify for extended due dates. To verify if your geographic area has been designated as a disaster area and to find applicable filing extension dates, consult the following official resources:

Keep Your Business Compliant This September

Meeting these September deadlines is a fundamental part of maintaining your business's compliance and financial health. If you need assistance preparing your extended returns, calculating estimated payments, or understanding how disaster declarations impact your filing timeline, please reach out to schedule a consultation.

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