2026 Illinois State Tax Deadlines
Illinois Individual income tax filing deadlines and extensions for 2026
If you are a resident or have been involved in business operations within Illinois, you might be required to file Illinois State taxes. Illinois follows a flat income tax structure with a 4.95% rate, making it essential to understand the specific deadlines and requirements for 2026.
Illinois business tax deadlines for 2026
C Corporations' deadlines and requirements
C Corporations in Illinois must file and pay Corporate Income Tax by April 15, 2027, following the federal due date schedule.
Filing requirements include:
- Complete Form IL-1120 (Illinois Corporation Income Tax Return)
- File via MyTax Illinois
- Payment must be made by April 15, 2027
- Mail submissions must be postmarked by the due date
Extension to file taxes in Illinois for 2026
The Extension Deadline for Illinois Corporate income tax returns is October 15, 2027. Illinois automatically grants a six-month extension following the federal extension timeline without requiring separate forms. This is an extension to FILE, not an extension to PAY—payment is still due April 15, 2027.
Pass-through entities
Illinois S Corporations and Partnerships tax deadlines for 2026
S Corporations and Partnerships must file in Illinois by March 15, 2027.
Required forms include:
- Form IL-1120-ST—S Corporation Return
- Form IL-1065—Partnership Return
Extension to file S Corporations and Partnerships taxes in Illinois for 2026
The Extension Deadline for Illinois S Corporations and Partnerships returns is September 15, 2027. Illinois automatically grants a six-month extension without requiring separate applications. This is an extension to FILE, not an extension to PAY—payment is still due March 15, 2027.
Illinois Individual tax considerations for 2026
Filing requirements and income thresholds
Illinois Individuals must file a State income tax return if their gross income exceeds specific thresholds based on filing status.
The flat tax rate for 2026 is:
- 4.95% on all Illinois taxable income
Estimated tax deadline for Illinois 2026
Estimated tax payments are due on the following schedule for calendar year filers:
- April 15, 2026
- June 15, 2026
- September 15, 2026
- January 15, 2027
Who must make estimated payments
You must make Illinois estimated income tax payments during 2026 if you expect to owe a significant amount in Illinois income tax after subtracting withholding and credits. Electronic payments are encouraged through the Illinois online tax system.
Individual tax deadline for Illinois 2026
Individual income tax returns are due April 15, 2027.
Available forms include:
- Form IL-1040—Individual Income Tax Return
- Form IL-1040X—Amended Individual Income Tax Return
Extension to file taxes in Illinois for 2026
The Extension Deadline is October 15, 2027, to file your Illinois Individual Income tax return. Illinois automatically grants a six-month extension without requiring separate extension forms. This is an extension to FILE, not an extension to PAY—payment is still due April 15, 2027.
What Illinois tax professionals need to know
Several key considerations make Illinois's tax landscape unique for 2026. Illinois's flat income tax system provides simplicity, but the state faces ongoing fiscal challenges that may affect tax policy. Electronic filing requirements continue to expand, and Illinois offers various tax planning opportunities despite budget constraints.
Electronic filing requirements
Illinois has implemented electronic filing systems for taxpayers:
- C Corporations and pass-through entities should file electronically through MyTax Illinois
- Individuals can use Illinois's online system for free electronic filing
- Business deductions can be processed more efficiently through electronic systems
- Professional preparers must use approved software vendors
Illinois's unique tax features for 2026
4.95% flat rate on all Illinois taxable income
Illinois provides simplicity with its flat tax structure.
No deduction for federal taxes paid
Unlike some other states, Illinois does not allow the deduction of federal taxes paid.
Local income taxes may apply in certain municipalities
Some Illinois municipalities impose local income taxes that require additional compliance.
Personal property replacement tax on businesses
Illinois imposes this tax on businesses as a replacement for the personal property tax on business equipment.
Local Income Tax Considerations
Some Illinois municipalities impose local income taxes:
- Chicago imposes a local income tax
- Other municipalities may have local income tax ordinances
- Compliance requirements vary by jurisdiction
- Individuals must consider multiple tax jurisdictions
Personal Property Replacement Tax
Illinois imposes a Personal Property Replacement Tax on businesses:
- Replaces the personal property tax on business equipment
- Applies to C Corporations, S Corporations, and Partnerships
- Different rates apply to different entity types
- Separate filing and payment requirements
State fiscal challenges
Illinois faces ongoing fiscal challenges that may affect tax policy:
- Potential for tax rate changes
- Budget pressures may affect credits and deductions
- Compliance enforcement may be enhanced
- Planning should consider potential policy changes
Business incentives and credits
Despite fiscal challenges, Illinois provides various business tax incentives:
- Economic Development for a Growing Economy (EDGE) credits
- Research and development credits
- Small business job creation credits
- Film production credits
Retirement income considerations
Illinois provides certain retirement income benefits:
- Military retirement pay exemptions may apply
- Social Security benefits are generally exempt from the Illinois state income tax
- Pension income considerations for residents
Professional tax guidance
For complex Illinois tax situations, consulting with qualified tax professionals through an accounting firm directory can help ensure compliance and identify available tax-saving opportunities.
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The material discussed on this page is meant for general illustration and/or informational purposes only and is not to be construed as investment, tax, or legal advice. You must exercise your own independent professional judgment, recognizing that advice should not be based on unreasonable factual or legal assumptions or unreasonably rely upon representations of the client or others. Further, any advice you provide in connection with tax return preparation must comply in full with the requirements of IRS Circular 230.
Please note that if any due date falls on a federal or state holiday or weekend, it may be adjusted to the next business day. Always verify current deadlines with the appropriate tax professional and authorities.
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