Illinois has a calendar quirk that catches preparers: its S-corporations and partnerships file on different dates. For the 2025 tax year, individual, C-corporation, and partnership returns are due April 15, 2026, but the S-corporation return is due March 16, 2026. Illinois also layers a Personal Property Replacement Tax on every business entity on top of the flat income tax.

What are Illinois's 2026 tax filing deadlines?

For the 2025 tax year, the Illinois individual return (IL-1040), C-corporation return (IL-1120), and partnership return (IL-1065) are due April 15, 2026. The S-corporation return (IL-1120-ST) is due March 16, 2026, since its statutory 15th-day-of-the-third-month date falls on a Sunday. Within Illinois, the S-corporation files in March while the partnership files in April.

What is the Personal Property Replacement Tax?

Illinois charges a Personal Property Replacement Tax on top of income tax. C-corporations pay 2.5%, for a combined 9.5% with the 7% corporate income tax. Partnerships, S-corporations, and trusts pay 1.5%. Every Illinois business entity owes the replacement tax, so it belongs in the liability projection even for pass-throughs that owe no entity income tax.

How does Illinois's extension work?

Illinois grants an automatic six-month extension for individuals, to October 15, 2026, with no form; pay any balance by April 15, 2026 using Form IL-505-I. Corporations get an automatic seven-month extension. The extension is to file only, so tax is due on the original date.

What is Illinois's pass-through entity (PTE) tax election?

Illinois lets a partnership or S-corporation elect to pay state tax at the entity level at 4.95%, which moves the deduction above the federal SALT cap. The election is annual, made on Schedule B of the IL-1065 or IL-1120-ST, and the Illinois legislature removed its expiration date, so it is available for 2025 and ongoing. Members receive a credit equal to 4.95% of their distributive share.

How does Illinois tax nonresident pass-through owners?

The entity withholds. An S-corporation, partnership, or trust must make Illinois income and replacement tax payments on behalf of nonresident owners, reported on Schedule K-1-P, unless the owner files Form IL-1000-E to certify an exemption. The withholding is part of the entity return.

How does a firm handle Illinois at scale?

The thing to get right is the split. A workflow that batches all Illinois pass-throughs together will mis-date either the March S-corporation return or the April partnership return, because Illinois puts them in different months. Track the IL-1120-ST and IL-1065 dates separately, carry the replacement tax into every business projection, and decide the PTE election before March. See how we think about scaling tax prep workflows.

Where do these dates come from?

All deadlines and thresholds here trace to the Illinois Department of Revenue. Verify against the source before filing.