Illinois begins accepting individual income tax returns on Monday, January 26, 2026, the same day the IRS opens the federal filing season.1Illinois Department of Revenue. Illinois Department of Revenue Announces Jan. 26 Start to 2026 Income Tax Season From that date, the Illinois Department of Revenue will process Form IL-1040 returns through MyTax Illinois, approved tax software, and paper filings. The deadline to file is April 15, 2026.
Why the Season Opens on January 26
Illinois synchronizes its opening with the IRS so that federal and state returns can move through tax software in a single pass. Your federal adjusted gross income from line 11 of Form 1040 is the starting figure on IL-1040, so the state cannot meaningfully process a return before the federal system is live.2Internal Revenue Service. Form 1040
Tax software providers often let you prepare and submit a return before January 26. Those submissions sit in a queue. The Department of Revenue will not begin processing them until the official opening date, so an early submission does not translate into an earlier refund.3Illinois Department of Revenue. 2026 Tax Filing Season Begins Today for Individuals – Forms Now Available
The April 15, 2026 Filing Deadline
Calendar-year taxpayers — which is nearly everyone filing an individual return — must file IL-1040 by April 15, 2026.1Illinois Department of Revenue. Illinois Department of Revenue Announces Jan. 26 Start to 2026 Income Tax Season April 15 falls on a Wednesday in 2026, so there is no weekend or holiday shift. In years when the 15th lands on a weekend or legal holiday, the deadline rolls to the next business day.
Fiscal-year filers run on a different clock. Their return is due by the 15th day of the fourth month after the end of their tax year.4Internal Revenue Service. When to File
The Automatic Extension to October 15
If April 15 arrives and your paperwork is not ready, Illinois grants an automatic six-month extension that moves the filing deadline to October 15, 2026.5Illinois Department of Revenue. Due Date/Extension to File Income Tax Return You do not need to file a request, submit a form, or ask permission. The extension is yours by default.6LII / Legal Information Institute. Illinois Admin Code Title 86 100.5020 – Extensions of Time for Filing Returns
Here is the catch. The extension covers the return, not the money. Any tax you owe is still due by April 15. If you pay after that date, penalties and interest start running from April 15 regardless of your October filing date.5Illinois Department of Revenue. Due Date/Extension to File Income Tax Return
If you expect a balance due and know you will file late, use Form IL-505-I (Automatic Extension Payment) to send what you can by April 15.7Illinois Department of Revenue. Payment Options for Individuals Then submit the complete IL-1040 any time before October 15.
Estimated Tax Dates Throughout 2026
If you are self-employed, draw significant income without withholding, or otherwise expect to owe at least $500 in Illinois income tax after credits, you make quarterly estimated payments on Form IL-1040-ES. The four 2026 due dates are:8Illinois Department of Revenue. 2026 IL-1040-ES Estimated Income Tax Payments for Individuals
- First payment: April 15, 2026
- Second payment: June 15, 2026
- Third payment: September 15, 2026
- Fourth payment: January 15, 2027
You can pay the full year’s estimate with the first installment or split it into four equal amounts across the dates above. Underpayment can trigger a separate penalty calculated on Form IL-2210.
What Happens If You Miss April 15
Two separate charges apply when April 15 passes without a filing or a payment. One is a late-filing penalty, the other a late-payment penalty. Both are calculated under the Illinois Uniform Penalty and Interest Act.9Illinois General Assembly. 35 ILCS 735/3-3
The late-filing penalty applies if you do not file by the due date, with an additional amount added if you still have not filed after the Department sends a nonfiling notice. The late-payment penalty runs from the original due date until you pay the balance. On top of both, interest accrues on unpaid tax at an annual rate of 7% through at least June 30, 2026.10Illinois Department of Revenue. Interest Rates
If you cannot pay the full balance by April 15, file anyway. Filing on time eliminates the late-filing penalty entirely and limits your exposure to late-payment charges and interest on the unpaid portion. The automatic six-month extension removes the late-filing risk without any action on your part, but it does nothing for the late-payment side.
How to File Once the Season Opens
Electronic Filing
The fastest route is MyTax Illinois, the state’s free online portal.11Illinois Department of Revenue. File Form IL-1040 Individual Income Tax Return on MyTax Illinois After you review and submit, the system issues a confirmation number as proof of filing. Electronic submission combined with direct deposit produces the quickest refund turnaround — typically about four weeks.12Illinois Department of Revenue. Where’s My Refund? Commercial tax software is the other common electronic route.
Paper Filing
Paper returns go to one of two Springfield addresses depending on whether you are enclosing a payment:13Illinois.gov. Mailing Addresses – Illinois Department of Revenue
- Without payment: PO Box 19041, Springfield, IL 62794-9041
- With payment: PO Box 19027, Springfield, IL 62794-9027
Paper returns generally take four to eight weeks to process. Sending to the wrong address can add weeks to that window, so verify before mailing.
Tracking Your Return After You File
Once your return is in, the “Where’s My Refund?” tool on the Illinois Department of Revenue website shows its status.12Illinois Department of Revenue. Where’s My Refund? Direct deposit shortens the final step compared to a mailed paper check. Expect roughly four weeks after an electronic filing and four to eight weeks after a paper filing before a refund arrives, assuming the return does not require additional review.
Mark January 26 for when the season opens, April 15 for when the return and any payment are due, and October 15 as your automatic fallback for the paperwork alone. Those three dates cover the full calendar for most Illinois individual filers in 2026.