September 2026 Business Tax Deadlines: What You Need to Know

September brings several important tax deadlines for businesses, including extended returns for S corporations and partnerships, corporate estimated tax payments, and employer payroll tax deposits. At MJ Ahmed CPA PLLC, we help businesses across the Dallas-Fort Worth area and beyond keep their filings and payments on track. Review the key due dates below to ensure your business remains compliant this month.

Professionals discussing business tax deadlines

September 15: S Corporations

If you requested an automatic six-month extension, you must file your 2025 calendar year income tax return (Form 1120-S) and pay any tax due by this date. Additionally, you are required to provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a substitute Schedule K-1. If applicable, you must also provide Schedule K-3 (Form 1120-S) or a substitute.

September 15: Corporations

Calendar year corporations are required to deposit the third installment of estimated income tax for 2026.

September 15: Partnerships

For partnerships that timely requested an automatic six-month extension, the 2025 calendar year return (Form 1065) is due. You must also furnish each partner with a copy of their Schedule K-1 (Form 1065) or a substitute Schedule K-1, along with Schedule K-3 (Form 1065) or its substitute, if applicable.

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

If you are an employer subject to the monthly deposit rules, September 15 is the deadline to deposit Social Security, Medicare, and withheld income tax for August 2026. This is also the due date for the nonpayroll withholding deposit for August 2026 if the monthly deposit rule applies.

September 30: Fiduciaries of Estates and Trusts

Fiduciaries who were granted a 5½-month extension must file their 2025 calendar year return (Form 1041) by September 30. If applicable, you must also provide each beneficiary with a copy of their Schedule K-1 (Form 1041) or a substitute.

Weekends and Legal Holidays

If any tax due date falls on a Saturday, Sunday, or legal holiday, the deadline is automatically extended to the next business day that is not itself a legal holiday.

Disaster Area Extensions

When a geographical area is officially designated as a disaster area, tax due dates are typically extended. To check if your location has been designated as a disaster area and to find information on filing extension dates, visit the following resources:

Stay Ahead of Your Fall Filing Requirements

Navigating extended returns and estimated payments requires careful attention to detail. Missing a deadline can result in unnecessary penalties that impact your bottom line.

With over 25 years of experience helping clients in the Dallas-Fort Worth area and globally, MJ Ahmed CPA PLLC is here to help you manage these critical dates. Contact our team today to ensure your business filings are accurate and submitted on time.

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