S Corporation Tax Return Due Date and 2026 Filing Rules

SMALL BUSINESS OWNERS

Anastasia Mironova EA

9/15/20265 min read

An S corporation does not pay federal income tax at the entity level, but it still has to file. The return reports the corporation's income, deductions, and credits and passes each shareholder's share through on Schedule K-1. Missing the filing date creates avoidable problems, so it pays to know exactly when Form 1120-S is due, how the calendar shifts from year to year, and what happens when the usual date lands on a weekend.

The Short Answer on the S Corporation Filing Deadline

The S corporation tax return is due by the 15th day of the third month after the end of the corporation's tax year. For a calendar-year S corporation, that falls on March 15 in most years. When the due date lands on a weekend or a holiday, the deadline to file and pay without penalty is extended to the next business day.

That adjustment is already in play for the current cycle. Form 1120-S and the related Schedule K-1 forms for the 2025 tax year are due Monday, March 16, 2026, because the typical March 15 date falls on a weekend.

Key Deadlines:

S-corp (calendar year) - Form 1120S -Due 15th day of the 3rd month, typically March 15th.

C-corporation (calendar year) - Form 1120- Due 15th day of the 4rd month, typically April 15th.

Why S Corporations File in March and C Corporations File in April

The two corporate deadlines are easy to mix up, and the confusion usually comes from the word corporation. A corporate income tax return is generally due by the 15th day of the fourth month following the end of the corporation's tax year, which is the rule most often associated with C corporations and lands on April 15 for a calendar-year filer. S corporations follow a different schedule: the 15th day of the third month after the tax year ends.

That one-month gap exists for a practical reason. S corporation income flows through to the shareholders, and the entity files earlier so owners have their Schedule K-1 in hand while they prepare their own returns. The IRS itself notes that some business entities have due dates other than April 15, and that the instructions for each type of form spell out the appropriate date.

Fiscal Year S Corporations Follow the Same Formula

Not every S corporation uses the calendar year. When a corporation has a fiscal tax year, the same rule applies against a different year-end. Count three months forward from the last day of the tax year and land on the 15th day of that third month.

Key Deadlines:

Fiscal year S-corp (Tax year ending March 31) is due on June 15th

Fiscal year S-corp (Tax year ending June 30) is due on September 15th

Fiscal year S-corp (Tax year ending September 30) is due on December 15th

The weekend and holiday rule still applies to those dates, so a deadline that falls on a Saturday, Sunday, or federal holiday moves to the next business day. If you are not certain which tax year your corporation uses, check the return filed last year or ask the preparer who handled it. Business entity return due dates are not uniform, and the form instructions are the authority for the specific date.

What an S Corporation Reports on Form 1120-S

Form 1120-S, U.S. Income Tax Return for an S Corporation, is the return filed with the IRS. It carries the corporation's income and deductions and produces the shareholders' Schedule K-1, which reports each owner's share of income, deductions, and credits. The IRS publishes the form along with current instructions and any recent updates, so the version to use can always be confirmed there.

A handful of details routinely slow down S corporation filings:

Shareholder information that does not match IRS records, including names, addresses, and taxpayer identification numbers.
Missing or late Schedule K-1 data from other entities in which the S corporation holds an interest.
Reasonable compensation questions for shareholder-employees, which sit outside the return itself but change the numbers that flow into it.
Ownership percentage and basis records that need to be reconciled before K-1s can be issued.

Working through those items in January and February is far easier than trying to resolve them in the final week before the March deadline.

Filing an Extension for an S Corporation Return

An S corporation can ask for more time. Form 7004 may be filed to obtain an automatic six-month extension of time to file the return. The request has to go in by the original due date, not after it passes.

State S Corporation Deadlines: California and Beyond

Federal and state deadlines do not always line up, and a state may or may not follow the federal weekend rule. California is a clear example. The state's income tax calendar lists March 16, 2026 as the date that 2025 S Corporation Income Tax returns and the tax due are required for calendar-year filers, reported on Form 100S.

Other states set their own terms. South Carolina ties the S corporation return and payment to the 15th day of the third month after the end of the business year, matching the federal cadence. Because every state can differ, check the revenue department for each state where the corporation is registered to do business, and confirm whether that state grants its own extension.

A Practical Filing Checklist for S Corporation Owners

Confirm your tax year. Most S corporations use the calendar year, which makes the mid-March deadline the one to plan around.
Gather shareholder data early. K-1 preparation is usually the last step in the process, and it depends on clean ownership records.
Decide on an extension before the deadline, not after. Form 7004 has to be filed by the original due date.
Check the state calendar. California's 2025 S corporation return and tax are also due March 16, 2026.
Keep the shareholder filing calendar in view. Owners use the K-1 to complete their own returns, so a late K-1 ripples outward.
Make sure your bookkeeping is up to date; if not, contact a professional bookkeeper as soon as possible.
If you plan to utilize professional services to file an S-Corp return, reach out to your EA, CPA, or tax practitioner early. Many firms require onboarding at least 4 weeks before the deadline due to high demand.

Where to Confirm the Current Deadline

Tax dates move. A weekend, a federal holiday, or a change in the filing calendar can push an S corporation deadline by a day or more, which is exactly what happened with the March 16, 2026 date for 2025 returns. The IRS instructions for Form 1120-S, the IRS small business FAQ pages, and your state revenue department are the places to verify the date that applies to your corporation before you rely on it.

Frequently Asked Questions

What is the tax return due date for an S corporation?

An S corporation return is due by the 15th day of the third month after the end of the corporation's tax year. For a calendar-year S corporation, that is usually March 15. When the date falls on a weekend or holiday, it moves to the next business day, which is why 2025 returns are due March 16, 2026.

Can an S corporation get an extension to file?

Yes. An S corporation can file Form 7004 to obtain an automatic six-month extension of time to file the return. The request must be submitted by the original due date.

What form does an S corporation file?

S corporations file Form 1120-S, U.S. Income Tax Return for an S Corporation. The return reports the corporation's income, deductions, and credits for the year, and it generates Schedule K-1 for each shareholder. The IRS publishes the form, its instructions, and any recent updates on its website.

Is the deadline different for a fiscal year S corporation?

It works the same way, just against a different year-end. Count three months from the last day of the tax year and use the 15th day of that third month. A fiscal year ending June 30 generally produces a September 15 deadline, and the weekend or holiday rule still applies.

Do state S corporation deadlines match the federal date?

Not always. California lists March 16, 2026, as the due date for 2025 S Corporation Income Tax returns and tax due for calendar-year filers. South Carolina ties its S corporation return and payment to the 15th day of the third month after the business year ends. Check each state where your corporation is registered.