"2026 return" can mean two things. It can be a return for a tax year ending in 2026, or a return filed during calendar 2026 for an earlier year. The final 2025 form (R-12/25) covers years ending on or after December 31, 2025 and before December 31, 2026. IDOR posted 2026 forms as drafts on September 1, 2026, and says not to file on drafts.
This is a preparation guide, not a replacement for IDOR's final instructions. Before submitting, verify the form revision, schedules, filing method, payment rules, and mailing address.
Key Takeaways
- Confirm the entity files IL-1120-ST, not IL-1120 or IL-1065.
- Reconcile federal 1120-S figures with Illinois additions, subtractions, apportionment, credits, and PTE items.
- Treat the filing deadline and payment deadline separately; the extension covers filing only.
- Use current IDOR materials and keep payment and filing confirmations.
Who Must File Form IL-1120-ST for 2026?
Under the 2025 instructions, you file IL-1120-ST if you are an S corporation under IRC Section 1361(a) with Illinois net income or loss. You also file if you are qualified to do business in Illinois and required to file federal Form 1120-S, regardless of income or loss.
S corporations generally don't pay Illinois income tax; shareholders do. The entity does owe replacement tax of 1.5% on income taxable to Illinois. Federal S status is required, so confirm the election is in effect before anything else.
Which Illinois return applies?
| Entity classification | Illinois return |
|---|---|
| S corporation | IL-1120-ST |
| Other corporation (C corporation) | IL-1120 |
| Partnership | IL-1065 |
An S corporation can be a member of a unitary group and apportion income on that basis, but it cannot file a combined return. If entity classification is unclear, check IDOR's current form directory.
What information should be gathered before preparation?
Pull these together first:
- Final federal Form 1120-S with all schedules
- Trial balance and year-end financial statements
- FEIN, legal name, and tax-year dates
- Shareholder names, addresses, residency, and ownership percentages
- Illinois and everywhere sales figures
- Estimated, extension, and PTE payments made
- Credits, carryforwards, and prior-year overpayments
Foreign-owned, multistate, or internationally managed businesses should not assume federal treatment settles the Illinois question. Review Illinois nexus, sales sourcing, and nonresident shareholder rules separately. The 2025 package has no separate guidance for foreign owners, so the standard rules and federal S eligibility apply.
How to Complete Form IL-1120-ST: Section-by-Section Instructions
The line map below comes from the final 2025 form. Final 2026 numbering isn't confirmed, so check each line against the form you actually file.
| Step | Lines | Function |
|---|---|---|
| 1 | Lettered items | Identification; item L is the PTE election |
| 2 | 1-7 | Federal Schedule K income |
| 3 | 8-13 | Deductions; unmodified base income |
| 4 | 14-22 | Illinois additions |
| 5 | 23-35 | Subtractions; line 35 is base income |
| 6 | 36-46 | Allocation and apportionment |
| 7 | 47-51 | Adjustments, net loss deduction, net income |
| 8 | 52-56 | Replacement tax, recapture, credits |
| 9 | 57-64 | Surcharges, withholding, PTE income and tax, penalty |
| 10 | 65-71 | Payments, refund or credit, balance |
Taxpayer identification and filing status
Enter the legal name, full FEIN, address, tax year, and accounting period exactly as on your federal return. IDOR flags partial FEINs and wrong-year forms, and both delay processing.
Match the Illinois period to the federal 1120-S period. Fiscal-year and final-return situations need extra care. Short-year rules are not clearly set out as a separate exception, so read the instructions for your situation before filing.
An amended return is not a checkbox on the original form. Use IL-1120-ST-X, covered later.
Federal income and Illinois adjustments
The return starts from federal Schedule K income. From there, Illinois additions come first (lines 14-22), then subtractions (lines 23-35), producing base income on line 35.
Research these categories in the current instructions:
- Special depreciation adjustments (Form IL-4562)
- Other additions and subtractions for businesses (Schedule M)
- Related-party expense add-backs (the 80/20 schedule)
- Exempt income and other modifications
- Gains on property acquired before August 1, 1969 (Schedule F)
Tie every adjustment to a ledger entry, the federal return, or a workpaper. If you attach federal Schedule D (1120-S), it goes with Schedule F. Any worked example you build should be labeled hypothetical, using figures checked against the official guidance.
Illinois income, replacement tax, and PTE considerations
Apportionment uses a single sales factor. Line 42 is Illinois sales divided by everywhere sales, rounded to six decimals. Finnigan apportionment applies to years ending on or after December 31, 2025. Special rules apply to certain industries.
Net income after the net loss deduction is multiplied by 1.5% on line 52. Check current-year rates and thresholds in the final instructions.
If the corporation elects pass-through entity (PTE) tax, note these points:
- The election is made annually on the return (item L)
- The rate is 4.95% of calculated PTE net income
- The election is irrevocable after the extended due date
- Schedule B reports PTE information, and K-1-P reports shareholder credits
- The election does not eliminate replacement tax
Non-electing S corporations generally owe pass-through withholding for nonresident shareholders who haven't provided Form IL-1000-E. Calculate it on K-1-P(3).
Shareholders, schedules, credits, and payments
Confirm schedule names and attachment rules in the final package. The 2025 set included:
- K-1-P: Partner's or Shareholder's Share of Income, Deductions, Credits, and Recapture
- B: Partners' or Shareholders' Information
- UB: Combined Apportionment for Unitary Business Group
- 1299-A: Tax Subtractions and Credits (for partnerships and S corporations)
- K-1-P(3): Pass-through Withholding Calculation for Nonresident Members
Common mistakes: using Schedule K-1-T (a beneficiary schedule you may receive from a trust or estate, not the shareholder schedule), and attaching 1299-D, whose attachment list doesn't include IL-1120-ST.
Complete K-1-P and K-1-P(3) first, then Schedule B Section B, then Section A. Then reconcile estimated payments, extension payments, withholding, prior-year credits, and overpayments to lines 65-71.
Review, signature, and submission readiness
Before signing or transmitting, compare the return against the federal return, books, ownership records, and every schedule. Check that the return is signed and dated, and that the preparer section is complete.
IL-1120-ST-V is a payment voucher, not a return. It never replaces IL-1120-ST. A payment made electronically may leave you with separate filing and payment confirmations, so keep both.
2026 Filing Deadlines, Extensions, and Tax Payments
Original filing deadline
The due date is the 15th day of the third month after the tax year ends. Illinois follows the federal date. A calendar-year 2025 return was due March 16, 2026.
For a calendar tax year ending December 31, 2026, the draft 2026 instructions show March 15, 2027. That is draft only. Confirm it once IDOR finalizes the form, expected in early 2027. Fiscal-year filers count three months from their own year-end.
Automatic extensions
Illinois gives an automatic seven-month extension. Per IDOR's 2025 instructions, no application is needed, including no IL-505-B. Calendar 2025 returns could run to October 15, 2026. The draft 2026 equivalent is October 15, 2027.
The extension covers filing only. Replacement tax, withholding, and any elected PTE tax remain due by the original deadline. Estimate the balance and pay it by then.
Estimated payments and PTE payments
Without a PTE election, S corporations have no required estimates, though voluntary prepayments are allowed. With a PTE election, estimates are required when expected replacement plus PTE liability, after specified withholding and credits, exceeds $500, per IDOR's estimated payment guidance.
- Installments are due the 15th of months 4, 6, 9, and 12
- Weekend or holiday dates move to the next business day
- Penalty protection requires paying 90% of current-year or 100% of prior-year liability
- Form IL-2220 computes estimated-payment penalties
Keep each payment type separate: return payments, extension payments, estimates, PTE payments, and payments responding to an IDOR notice.
Penalties, interest, and late corrections
Publication 103 (September 2025) lists the following:
| Item | Rule |
|---|---|
| Late filing | Lesser of $250 or 2% of tax due, net of timely payments |
| Continued nonfiling | Additional greater of $250 or 2%, capped at $5,000 |
| Late payment | 2% at 1-30 days; 10% at 31+ days |
| Dishonored payment | $25 per remittance |
| Interest | Simple daily accrual; underpayment rate 7% through December 31, 2026 |
A return that IDOR can't process and you don't correct within 30 days of notice can draw the filing penalty. If you have a wrong FEIN, wrong period, or missed payment, correct it promptly and consider a professional review.
How to File, Pay, and Mail Form IL-1120-ST
Electronic filing options
IDOR directs return filing through approved software using IRS Modernized e-File (MeF). MyTax Illinois handles payments and account management, not IL-1120-ST preparation. Filing and payment are separate steps.
- Federally mandated e-filers must e-file the equivalent Illinois return
- Annual liability of $20,000 or more requires electronic payment
- MeF does not support amended returns, and the supported list excludes unitary filings
- A rejected return filed by the extended deadline gets a 10-calendar-day perfection period
- Payment requests tied to a rejected return are not processed
Making an IL-1120-ST payment online
- Choose the channel: MyTax Illinois, MeF debit, ACH credit, or credit card.
- Select the correct tax type and payment purpose (original return, extension, estimate, or PTE).
- Enter the FEIN and tax period exactly.
- Enter the amount and withdrawal date.
- Save the confirmation number.
Channel notes:
- MyTax ACH debit: register or log in, then supply bank routing and account numbers.
- MeF debit: the requested date requires receipt at least one banking business day earlier.
- ACH credit: request instructions through MyTax; they arrive about 7-10 business days after IDOR processes the request. EFT-1 can't be used.
- Credit card: Illinois E-Pay charges 2.25%, minimum $1.
Mailing the return
Use the address printed on the current form package. For reference, the 2025 form listed two:
| Mailing scenario | Address |
|---|---|
| Payment enclosed | Illinois Department of Revenue, P.O. Box 19053, Springfield, IL 62794-9053 |
| No payment enclosed | Illinois Department of Revenue, P.O. Box 19032, Springfield, IL 62794-9032 |
Never reuse an address from a prior-year filing. Use tracked delivery, sign the return, and keep a complete copy.
Form IL-1120-ST-V and other payment records
The voucher accompanies non-electronic estimated, extension, pass-through, and original-return payments. Don't mail it if no payment is due or you paid electronically.
When paying by mail:
- Make checks payable to Illinois Department of Revenue
- Write the FEIN, the tax year ending, and "IL-1120-ST-V" on the check
- Keep the voucher, payment record, confirmation, and completed return together
- Use IL-1120-ST-X-V for amended returns
Common Errors and Final Submission Checklist
The errors IDOR sees most:
- Choosing the wrong Illinois return
- Using a draft or prior-year form
- Mismatched FEIN or tax period
- Missing schedules, or an incomplete Schedule B
- Federal and Illinois figures that don't reconcile
- Confusing an extension with extra time to pay
- Mailing payment to an outdated address
If you need to amend, file IL-1120-ST-X after a processable original. File a separate form for each year, include an explanation and support, and never resubmit the original form with new figures. After a federal change that raises Illinois tax, amend and pay within 120 days.
Final checklist:
- Name, FEIN, and tax-period dates match the federal return.
- Federal 1120-S income is reconciled to Illinois additions and subtractions.
- Apportionment is computed and rounded properly.
- K-1-P, K-1-P(3), and Schedule B are complete and consistent.
- PTE election and estimated payments are reflected correctly.
- The return is signed, or e-file validation passed.
- Payment confirmation or voucher is saved.
- Mailing evidence and complete copies are retained.
VJM Global assists US-based and internationally owned businesses with accounting, tax compliance coordination, and financial documentation. Confirm final filing requirements directly with IDOR or a qualified tax professional.
Frequently Asked Questions
What are the instructions for filing Form IL-1120-ST?
Confirm the entity is an eligible S corporation, reconcile federal 1120-S figures to Illinois, and attach the required schedules. Use the current-year IDOR instructions, file through approved software, and pay replacement tax by the original due date.
Where do I mail Form IL-1120-ST?
Use the address printed on the current-year form or instructions. The payment and no-payment addresses differ, so choose the one that matches your return.
How do I make an IL-1120-ST payment online?
Use MyTax Illinois, MeF debit, ACH credit, or credit card. Select the correct tax type and period, enter the FEIN, and save the confirmation number.
Who is required to file Form IL-1120-ST for 2026?
Qualifying S corporations with Illinois income or loss, or those qualified to do business in Illinois and required to file federal 1120-S, may need to file. Federal classification and Illinois activity or nexus drive the answer.
What is the due date for the 2026 Form IL-1120-ST?
It is the 15th day of the third month after year-end. A calendar 2026 return shows March 15, 2027 in the draft instructions; a return filed during 2026 for calendar 2025 was due March 16, 2026.
Does an extension give more time to pay IL-1120-ST tax?
No. The seven-month extension covers filing only. Replacement tax, withholding, and elected PTE tax are still due by the original deadline, and interest and penalties can apply to late payments.
