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Essential September 2025 Business Filing Deadlines

September 2025 is a pivotal month for business tax obligations, with critical due dates for entities such as S corporations and partnerships that have applied for extensions, alongside corporate estimated tax payments and payroll tax deposits set for mid-September.

September 15 - S Corporations
Ensure the filing of your 2024 calendar year income tax return (Form 1120-S), and remit any taxes due, applicable only if you have secured a standard 6-month extension. Shareholders should receive their Schedule K-1 (Form 1120-S) copies, or a substitute, as well as a Schedule K-3 (Form 1120-S) if required.

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September 15 - Corporations
Corporations must deposit their third installment of estimated income tax for 2025 by this date, referencing their 2024 calendar year.

September 15 - Partnerships
Partnerships with a 6-month filing extension should file their 2024 calendar year return (Form 1065), and provide all partners with Schedule K-1 (Form 1065) or a substitute, in addition to Schedule K-3 (Form 1065) when it's pertinent.

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September 15 - Social Security, Medicare, and Withheld Income Tax
If you are an employer subject to the monthly deposit rules, ensure your payment for Social Security, Medicare, and withheld income tax for August 2025 is submitted by September 15. This deadline also encompasses nonpayroll withholding deposits for August under the monthly deposit rule.

Weekends & Holidays
Should any due date fall on a weekend or legal holiday, the IRS grants an automatic extension to the next operational business day that isn't a public holiday.

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Disaster Area Extensions
Designations of geographical areas as disaster zones trigger extensions for tax obligations. Stay informed about such designations and relevant filing deadlines via the following resources:
FEMA: fema.gov/disaster/declarations
IRS: irs.gov/newsroom/tax-relief-in-disaster-situations

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