Critical Tax Deadlines for Businesses in September 2025

September 2025 is a crucial month for businesses keeping track of tax responsibilities. This period includes important deadlines for S corporations, partnerships, corporate estimated tax payments, and payroll tax deposits due by September 15.

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September 15 - S Corporations

Submit the 2024 calendar year income tax return (Form 1120-S) and settle any outstanding tax liabilities if you filed for an automatic 6-month extension. Additionally, ensure each shareholder receives their Schedule K-1 (Form 1120-S) or a substitute, and if required, Schedule K-3 (Form 1120-S) or its substitute.

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September 15 - Corporations

Corporations on a calendar year basis must pay the third installment of their estimated income tax for 2025 on or before this date.

September 15 - Partnerships

File your 2024 calendar year return (Form 1065) if you obtained a 6-month extension. Distribute Schedule K-1 (Form 1065) or a substitute to each partner, along with Schedule K-3 (Form 1065) or its substitute if needed.

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

Employers adhering to the monthly deposit rule must submit deposits for Social Security, Medicare, and withheld income tax for August 2025. This is also the submission deadline for the nonpayroll withholding deposit applicable under the monthly deposit rule.

Consideration of Weekends & Holidays

Should a designated due date coincide with a Saturday, Sunday, or a recognized legal holiday, the deadline extends to the subsequent business day that is not a holiday.

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Disaster Area Extensions

When an area receives a disaster designation, tax deadlines are extended automatically. For up-to-date information regarding disaster declarations and related filing extensions, please visit the following:

FEMA: https://www.fema.gov/disaster/declarations
IRS: https://www.irs.gov/newsroom/tax-relief-in-disaster-situations

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