Key Tax Deadlines for Businesses in September 2025

As September 2025 approaches, it’s crucial for businesses to stay on top of impending tax deadlines. The month is packed with important compliance requirements, including S corporation and partnership filings, corporate estimated payments, and payroll tax obligations, all due by September 15.

September 15 - S Corporations

S corporations must file a 2024 calendar year income tax return using Form 1120-S and settle any outstanding taxes. This deadline applies if a 6-month extension was previously requested. Shareholders should receive their Schedule K-1 (Form 1120-S) or substitute documents, along with Schedule K-3 or its substitute, if applicable.

September 15 - Corporations 

Calendar year corporations are required to deposit the third installment of their estimated income tax for 2025 by this date.

Image 1

September 15 - Partnerships

Partnerships should ensure the filing of a 2024 calendar year return with Form 1065, provided a 6-month extension was granted. Partners need copies of their Schedule K-1 (Form 1065) or substitutes, and, if relevant, Schedule K-3 (Form 1065).

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

If monthly deposit rules apply to you as an employer, ensure all August 2025 Social Security, Medicare, and withheld income tax deposits are made by this deadline. The same date applies for nonpayroll withholding deposits.

Image 2

Weekends & Holidays:

Should any due date fall on a Saturday, Sunday, or legal holiday, the deadline is extended to the next business day that is not a legal holiday. 

Disaster Area Extensions:

In the event of a disaster designation for any area, deadlines will be adjusted accordingly. For updated information, check these sites:

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

Image 3

Share this article...

Want tax & accounting tips and insights?

Sign up for our newsletter.

I confirm this is a service inquiry and not an advertising message or solicitation. By clicking “Submit”, I acknowledge and agree to the creation of an account and to the and .
Jeanie K's Tax and Accounting We'd Love To Chat!
Please feel free to use our Ai chat assistant or click the contact button below.
Please fill out the form and our team will get back to you shortly The form was sent successfully