Essential September 2026 Tax Due Dates for Businesses

September brings a crucial wave of tax deadlines for businesses, encompassing extended returns for S corporations and partnerships, corporate estimated tax payments, and employer payroll tax deposits. Keeping a close eye on these key due dates will help ensure your business filings and payments remain strictly on track.

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September 15: S Corporation Extended Returns

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

September 15: Corporate Estimated Tax Payments

For calendar year corporations, September 15 marks the deadline to deposit the third installment of your estimated income tax for the 2026 tax year.

September 15: Partnership Extended Returns

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

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September 15: Payroll and Withholding Deposits

If your business operates under the monthly deposit rules, you must make your deposit for Social Security, Medicare, and withheld income tax for August 2026 by September 15. This is also the standard due date for your August 2026 nonpayroll withholding deposit, provided the monthly deposit rule applies to your operations.

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September 30: Fiduciaries of Estates and Trusts

For those managing estates and trusts who were granted a 5½-month extension, September 30 is the deadline to file the 2025 calendar year return (Form 1041). If applicable, you must also supply each beneficiary with a copy of their Schedule K-1 (Form 1041) or an approved substitute.

Rules for Weekends, Holidays, and Disasters

When a standard tax due date falls on a Saturday, Sunday, or legal holiday, the IRS automatically extends the deadline to the next business day that is not a legal holiday.

Additionally, if your business is located in a designated disaster area, your deadlines may be extended. To check if your geographical area qualifies for a filing extension, visit the official declarations and relief pages:

Stay Ahead of Your Fall Tax Obligations

Missing a filing or payment deadline can result in unnecessary penalties and operational friction. By tracking these September milestones, you protect your business's financial health. If you need assistance preparing your extended returns, calculating estimated payments, or ensuring compliance with payroll deposits, schedule a consultation with our tax professionals today.

Let’s Start a Conversation.
You can count on us for professional guidance along with timely, and reliable tax services. If you’re ready to get started, or just want to start a conversation, then click below.
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