Illinois S corporation return

File the Illinois S corporation return with the Illinois Department of Revenue by the 15th day of the 3rd month after the fiscal year ends (March 15 for a calendar year), with the owners' state K-1s.

When: By the 15th day of the 3rd month after the fiscal year ends (March 15 for a calendar year).
Agency: Illinois Department of Revenue
Applies to: S corporations registered in Illinois.
Source: tax.illinois.gov, read October 22, 2026

Jay explains

An S corporation doing business in Illinois files a state information return by the 15th day of the 3rd month after the fiscal year ends (March 15 for a calendar year), alongside the federal return, so the owners can report their shares on their own Illinois returns. Some states charge a minimum tax or filing fee with it; check the Illinois Department of Revenue page.

Next dates

For a calendar year company. Shown as the agency states them; if a date falls on a weekend, file before it.

March 15, 2027
March 15, 2028

Also in Illinois

Illinois annual report and franchise tax Illinois annual report Illinois annual report Illinois nonprofit annual report Illinois charitable organization annual report Illinois corporate income tax return Illinois corporate estimated tax payment Illinois partnership return Illinois personal income tax return with business income Illinois unemployment insurance wage report Illinois W-2 filing with the Department of Revenue Illinois harassment prevention training

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What kind of organization is it?

An LLC's tax treatment decides which federal return it files. Choose the line that matches yours. Nonprofits are covered too.

State of formation

Employees

Count everyone on payroll. An S corporation's working owners are employees: they are paid a salary.

Where is it based? The state of the office, or of most of the people

A company based outside its state of formation is registered in both and files in both.

More about the company: other states, payroll, dates, abroad

Company name

Date registered in each other state

Optional. Some states key their report to the day you registered there rather than your date of formation; without it, the formation date stands in and the calendar says so.

Who runs payroll?

Fiscal year ends in

Date of formation

Illinois keys its report to this date. Add it and the calendar places it.

Annual revenue

Some taxes start at a revenue threshold.

We pay independent contractors We offer a retirement plan (401(k), SIMPLE, SEP)

Outside the United States We hold a bank or financial account outside the US A person or company outside the US owns 25% or more of it We own 10% or more of a company outside the US We pay people or companies outside the US

Each adds the FinCEN or IRS form that comes with it: the FBAR, Form 5472, Form 5471, Forms 1042 and 1042-S.

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