2027 Illinois Tax Deadlines

Last updated: July 29, 2026

Key Illinois tax dates at a glance

This block is the single date reference for 2027 Illinois filing, payment, estimated-payment, and extension planning.

• April 15, 2027: First Illinois estimated payment for calendar-year Individuals.
• June 15, 2027: Second Illinois estimated payment.
• September 15, 2027: Third Illinois estimated payment.
• January 18, 2028: Fourth Illinois estimated payment.
• March 15, 2028: Illinois S Corporation return due for calendar-year filers.
• April 18, 2028: Illinois Individual, C Corporation, and Partnership returns and balances due for calendar-year filers.
• October 16, 2028: Extended Illinois Individual, S Corporation, and Partnership returns due; payment is not extended.
• November 15, 2028: Extended Illinois C Corporation return due; payment is not extended.

When are Illinois taxes due in 2027?

Illinois 2027 calendar-year Individual, C Corporation, and Partnership returns are generally due April 18, 2028, while S Corporation returns are due March 15, 2028. Estimated payments and filing-only extensions should be tracked separately by entity type.

When are Illinois C Corporation taxes due for 2027?

Illinois C Corporations file Form IL-1120 by April 18, 2028, and pay the expected balance by April 18, 2028. Illinois C Corporations report income tax and Personal Property Replacement Tax with the same return. Under current IDOR guidance, the replacement tax rate is 2.5% for C Corporations.

What does the Illinois C Corporation extension change?

The extended filing deadline for a calendar-year Illinois C Corporation return is November 15, 2028. The extension gives more time to file, not more time to pay, so the expected balance remains due April 18, 2028.

When are Illinois S Corporation and Partnership taxes due for 2027?

Illinois S Corporations file Form IL-1120-ST and Partnerships file Form IL-1065 by March 15, 2028 for S Corporations and April 18, 2028 for Partnerships.

What pass-through payment rules should teams watch?

Illinois S Corporations receive an automatic seven-month extension, while Partnerships receive a six-month extension. Illinois replacement tax is currently 1.5% for S Corporations and Partnerships, and a PTE-tax election can add entity-level payments and owner-credit reporting. The projected extended filing deadline is October 16, 2028; payment and election deadlines are not automatically postponed.

When are Illinois Individual taxes due for 2027?

An Illinois Individual return using Form IL-1040 is due by April 18, 2028, and any remaining state income tax should be paid by April 18, 2028.

When is the Illinois Individual extension deadline?

The extended filing deadline for a 2027 Illinois Individual income tax return is October 16, 2028. The extension applies to filing only, so the payment deadline remains April 18, 2028.

When are Illinois estimated payments due?

Calendar-year Illinois estimated payments are due on this schedule:

• April 15, 2027.
• June 15, 2027.
• September 15, 2027.
• January 18, 2028.

Who must make Illinois estimated payments?

Taxpayers who expect an Illinois income-tax balance after withholding and credits may need estimated payments. Apply the tax-year 2027 threshold, safe-harbor, and voucher instructions when the filing-year guidance is released.

What Illinois rules should tax professionals watch for 2027?

Beyond the headline due dates, Illinois has entity-level replacement-tax and pass-through rules that should stay on the review checklist:

How does Illinois Personal Property Replacement Tax affect entity calendars?

Replacement tax is reported with the entity return, so payment controls should cover both income tax and replacement tax before the original due date.

Why do Illinois S Corporations and Partnerships have different original dates?

Illinois S Corporations use March 15, 2028, while Partnerships use April 18, 2028, even though both extended filing dates fall on October 16, 2028.

How should teams handle Illinois PTE tax?

A valid Illinois PTE-tax election can add entity-level estimates, schedules, and owner-credit reporting, so the election should be tracked separately from the filing extension.

Tax-professional context: Illinois return calendars, entity-level elections, owner reporting, withholding, and payment rules require a state-specific review. TY2027 forms and the final 2028 filing calendar remain subject to confirmation when the state publishes them.

Electronic filing requirements: Illinois uses MyTax Illinois for supported account management, filing, correspondence, and payment services. Use approved software for supported corporate, pass-through, and Individual returns. Use MyTax Illinois for eligible payments and account work. Review state electronic-filing mandates separately from federal requirements. Keep proof of timely electronic submission and payment with the workpapers.

Entity-specific extension periods: Illinois uses a seven-month extension for C and S Corporations, while Partnerships use a six-month extension.

What sources support these Illinois deadlines?

• Illinois DOR Corporation Income Tax.
• Illinois DOR S Corporation Income Tax.
• Illinois DOR Partnership Income Tax.
• Illinois Personal Property Replacement Tax.

Illinois tax deadlines for replacement tax

Illinois deadline work includes corporate and pass-through returns, replacement-tax awareness, Individual dates, and payment tasks that need a shared operating view. Instead's AI-driven tax platform keeps state rules connected to execution.

Use filing for Illinois return movement, planning to coordinate decisions before due dates, and resolution when notices or payment issues need cleanup. Built for tax professionals who want a better tax operation system to manage compliance and advisory work for clients.