Skip to main content
Employment Sign Up For Updates

Who must file Form IL-1120-ST and when is its due date?

You must file Form IL-1120-ST, Small Business Corporation Replacement Tax Return, if you are a small business corporation (S corporation) as defined in Internal Revenue Code (IRC) Section 1361(a) that

  • has net income or loss as defined under the Illinois Income Tax Act (IITA) or 
  • is qualified to do business in the state of Illinois and is required to file federal Form 1120S, U.S. Income Tax Return for an S Corporation (regardless of net income or loss).   

In general, Form IL-1120-ST is due on or before the 15th day of the 3rd month following the close of the taxable year. We grant you an automatic seven-month extension of time to file your small business corporate tax return.

See our Subchapter S (Small Business) Corporation web page for more information.                                                                                                                 

You may access additional information by searching the entire Illinois Department of Revenue website.

If you have additional inquiries, you may submit them on the Questions, Comments, or Requests form.