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Key September 2025 Tax Deadlines for Businesses

In September 2025, businesses face several critical tax deadlines, including extended filings for S corporations and partnerships, corporate estimated tax payments, and payroll tax deposits—all due by September 15.

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S Corporation Filings - September 15
File your S corporation 2024 calendar year income tax return using Form 1120-S, ensuring any due taxes are paid. This deadline is applicable if an automatic six-month extension was filed. Remember to distribute Schedule K-1 (Form 1120-S) or its substitute and, if needed, Schedule K-3 to each shareholder.

Corporate Estimated Tax - September 15
Corporations must deposit the third installment of their estimated income tax for the calendar year 2025 by this date.

Partnership Filings - September 15
For partnerships granted a six-month filing extension, the 2024 Form 1065 must be submitted by September 15. Partners should receive their Schedule K-1 (Form 1065) or substitute, along with Schedule K-3 where applicable.

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Social Security, Medicare, and Withholding Taxes - September 15
Businesses required to follow monthly deposit rules must submit their August 2025 deposits for Social Security, Medicare, and income taxes by this deadline. The same due date applies for nonpayroll withholding deposits, if the monthly rule applies.

Weekend and Holiday Adjustments
Deadlines falling on weekends or legal holidays automatically extend to the next operational day.

Disaster Area Filing Extensions
In designated disaster areas, extended deadlines apply. For current disaster declarations and applicable filing extensions, consult the following resources:
FEMA: https://www.fema.gov/disaster/declarations
IRS: https://www.irs.gov/newsroom/tax-relief-in-disaster-situations

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