IRS Form 1120-S is the tax form S corporations use to file a federal income tax return. This tax form informs the IRS of your total taxable earnings in a tax year. It’s used to determine an S corporation’s business income, gains, losses, tax credits, and deductions.

Every S corporation should file the 1120-S tax form, including LLCs taxed as S corporations. Employees or shareholders of an S corporation don’t need to file this form. Earnings should be reported on their individual federal tax return. Employees will receive a W-2 form from the corporation, while shareholders will receive a Schedule K-1 (Form 1120-S).

If you want to elect S corporation status, you should file Form 2553 to transition your business to an S corporation either during the corporation’s first tax year for which you want to make the election or within the first 75 days following that first tax year. Once you elect treatment as an S corporation, you should file the 1120-S tax form each year for federal tax purposes.

When do I file a 1120-S Tax Form?
The deadline is the 15th day of the third month after the end of your tax year. So, if your business’ tax year ends on December 31, your due date is on March 15 each year.  (This is unless it falls on a weekend or holiday, then it’s usually the Monday after.)