Illinois S Corporation Tax Deadlines 2026
Complete guide to S Corporation filing dates, extensions, estimated payment schedules, and required forms in Illinois for the 2026 tax year. Illinois imposes a state income tax, so S Corporation entities must file both federal and state returns.
Illinois S Corporation filing deadline
Calendar-year due date follows federal timing for pass-through entities.
Due: Mar 16, 2026
Extension: Sep 15, 2026
Form: State S-corporation return
Estimated payments
No state estimated payment schedule listed.
Due: N/A
Illinois S Corporation Tax Calendar at a Glance
| Event | Date |
|---|---|
| Filing deadline | Mar 16, 2026 |
| Extension deadline | Sep 15, 2026 |
FAQ
When is the S Corporation tax filing deadline in Illinois for 2026?
The Illinois S Corporation tax return is due Mar 16, 2026 for the 2026 tax year. An extension is available until Sep 15, 2026.
Does Illinois require S Corporation estimated tax payments?
Estimated payments are not typically required at the entity level for S Corporation in Illinois, but confirm with Illinois Department of Revenue.
What form do I file for a S Corporation in Illinois?
S Corporation entities in Illinois file using Form State S-corporation return. You can download the form from Illinois Department of Revenue at https://tax.illinois.gov.
Can I get an extension for my Illinois S Corporation tax return?
Yes. Illinois allows an extension for S Corporation returns until Sep 15, 2026. Note that an extension gives you more time to file, not more time to pay — any tax owed is still due by the original deadline of Mar 16, 2026.
What are the penalties for filing late in Illinois?
Illinois typically charges a failure-to-file penalty (usually 5% of unpaid tax per month, up to 25%) and a failure-to-pay penalty (usually 0.5-1% per month). Interest also accrues on unpaid balances. Filing for an extension by the original deadline avoids the failure-to-file penalty but not the failure-to-pay penalty.
Where can I file Illinois S Corporation taxes?
File through Illinois Department of Revenue. Visit https://tax.illinois.gov for e-filing options, downloadable forms, and payment portals.
Illinois Estimated Tax Payments
View the full quarterly estimated payment schedule for Illinois, including all entity types and payment methods.
View Illinois estimated payments →