Critical September 2025 Tax Deadlines for Businesses

As a business owner, it is imperative to stay ahead of tax deadlines to ensure compliance and avoid penalties. September 2025 is a crucial month for businesses, particularly those operating as S corporations and partnerships. Additionally, corporations need to address estimated tax payments, and employers must take care of their payroll tax responsibilities. This guide provides an overview of these important due dates.

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September 15 - S Corporations

If your S corporation opted for an automatic 6-month extension, you need to file your 2024 calendar year income tax return (Form 1120-S) and remit any taxes due by September 15. It is also essential to provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a suitable substitute, and if necessary, include a Schedule K-3 (Form 1120-S) or its alternative.

September 15 - Corporations

This date marks the deadline for depositing the third installment of the estimated income tax for 2025 for corporations that operate on a calendar year basis.

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September 15 - Partnerships

Partnerships that filed for a 6-month extension must submit their 2024 calendar year returns (Form 1065) by this date. Additionally, partners should receive their respective Schedule K-1 (Form 1065) or its substitute, along with Schedule K-3 (Form 1065) if applicable.

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

Employers adhering to the monthly deposit rules must deposit August 2025's Social Security, Medicare, and withheld income taxes by this deadline. The same applies to nonpayroll withholding deposits if the monthly filing rule is applicable.

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Considerations for Weekends & Holidays:

If a tax deadline falls on a weekend or legal holiday, it is automatically extended to the next business day that is not a holiday.

Disaster Area Extensions:

Taxpayers in disaster-designated areas may receive deadline extensions. For up-to-date information, visit the FEMA disaster declarations webpage and the IRS's tax relief page.

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