Key takeaways
- Form MO-1120S is due on the 15th day of the fourth month, so April 15, 2027 for a calendar 2026 year, a month after the federal return.
- The Missouri extension follows the federal Form 7004, but you must check the box on the MO-1120S and attach the 7004.
- Form MO-PTE is an annual election, not a default. It is worth modeling only when the owners cannot deduct state tax on their own returns.
- A nonresident shareholder with under $1,200 of Missouri assignable income, or one in a composite return, may fall outside the MO-1NR withholding filing.
Which Missouri filings a client owes
A federal S corporation files Form MO-1120S if it has a shareholder who is a Missouri resident or any income from Missouri sources. The 2025 instructions have you attach federal Form 1120-S with every Schedule K-1, plus any Form 8886. The corporation also needs a Missouri tax identification number, so ask for the registration letter first.
The MO-1120S is where the Missouri adjustments to federal income live. Page 3 allocates those adjustments to shareholders, and the corporation must give each shareholder a copy or an extract by the due date including extensions, with other shareholders' Social Security numbers and share percentages removed. Shareholders use Form MO-1040.
Two add-backs catch most files. Line 1a adds back income taxes of every state deducted on the federal return, and the instructions say that can include a state's elective pass-through entity tax. Line 1b adds back Kansas City and St. Louis earnings tax. The earnings tax figure sits in the payroll or tax ledger, not on the 1120-S.
Due date and extension for tax year 2026
Missouri's rule is the 15th day of the fourth month after year end. The 2025 instructions give the example of calendar 2025 due April 15, 2026. For calendar 2026 that is April 15, 2027. Federal Form 1120-S is due by the 15th day of the third month, which is March 15, 2027. The 2025 federal return was due March 16, 2026.
If the corporation has a federal extension, the Missouri time to file is automatically extended. Form 7004 extends federal filing by six months, which would run from March 15, 2027 to September 15, 2027. The Missouri instructions give no separate length, so confirm the date in the 2026 instructions before quoting it.
Check the box at the top of the MO-1120S for an approved federal extension and attach a copy of Form 7004. The instructions warn that failing to check the box may result in disallowing the extension.
Form 7004 does not extend the time to pay federal tax. For the MO-PTE, an extension to file does extend time to pay, but the entity must pay by the extended date to avoid a 5 percent addition to tax, and simple interest still runs from the original due date.
Nonresident shareholders: MO-1NR, MO-2NR and the exceptions
Every S corporation with nonresident individual shareholders must file Form MO-1NR, the Income Tax Withheld for Nonresident Individual Partners or S-Corporation Shareholders return, and send copies of Form MO-2NR to those shareholders. There are three exceptions in the 2025 instructions. The first is a nonresident not otherwise required to file who elects to have Missouri tax paid through the corporation's composite return. The second is a shareholder whose Missouri assignable federal adjusted gross income from the corporation is less than $1,200. The third is a liquidation or termination year with no cash or property distributed in the current and prior taxable year.
Worked example. A Kansas resident owns 4% of a Missouri S corporation, and the Missouri-assigned share of that income comes to less than $1,200 after apportionment. With no other Missouri return required, that shareholder falls under the second exception. A shareholder at exactly $1,200 does not, because the test is less than $1,200. Get the Missouri apportionment from Form MO-MSS before you decide, not the federal K-1 amount.
A nonresident can ask to be exempt from withholding by filing Form MO-3NR under section 143.411, RSMo. The timing differs between the two forms. MO-1NR is due with the S corporation return, including any extension. MO-3NR is due by the original due date, without regard to an extension. A client who extends the return but forgets that distinction loses the exemption.
The MO-PTE election: when it helps and how it is made
For tax years ending on or after December 31, 2022, an S corporation or an LLC taxed as one may elect to be an affected business entity on Form MO-PTE. A separate election is required each year, and once made it cannot be revoked for that year. The point is the federal cap on individual state and local tax deductions: entity-level tax is deducted on Form 1120-S, which lowers K-1 income.
The 2025 form computes tax at 4.7 percent of Missouri net income. The Department's FAQ lists 5.3% for 2022, 4.95% for 2023 and 4.8% for 2024. The 2026 rate was not published when checked, so confirm it before modeling. The base starts at Schedule K line 18 of Form 1120-S, then applies Missouri additions, subtractions and apportionment, and the form requires the Missouri business income deduction to be computed at the entity level on Schedule PTE-BD.
Mechanics matter more than the math. The return must be signed by each member, or by an authorized officer, manager or member under penalty of perjury, or by a designated affected business entity representative. Attach Form 2827 naming that representative. Without one, the instructions say the election is ineffective. The Department's FAQ says payment cannot be made electronically, so mail the check with Form MO-PTEV. Estimated payments are not required, though Form MO-PTEAP allows early payments.
Opt-outs, member credits and the percentage trap
A member may opt out of the entity tax. Nonresidents use Form MO-PTENR and agree to file a Missouri return and pay tax on the entity's income. Residents use Form MO-PTE Opt-Out. The opt-out must reach the Department on or before the earlier of the original MO-PTE due date or the actual filing date, and once filed it applies to later years until revoked. For 2025 the instructions also limited opt-outs to entities whose original due date was on or after August 28, 2025 and that had not yet filed. Confirm 2026.
Opt-outs change the other members' credit percentages. The instructions give this example: an opt-out holds 30% and another shareholder holds 10%, so the second shareholder's credit percentage becomes 14%, which is 10% divided by 70%. The opt-out member's items also come out of line 1 of the MO-PTE.
Members claim their share on Form MO-TC, attaching the K-1 and either page 4 of the MO-PTE, Form MO-5889 or an equivalent report. The credit is nonrefundable but carries forward indefinitely.
What to collect and what to send the client
Collect the federal Form 1120-S as filed or in final draft, every K-1, the Missouri tax ID letter, Form 7004 if extended, and Form 8886 if the client had a reportable transaction. For a multistate client, get gross receipts by state so you can complete Form MO-MSS. The Missouri receipts factor uses the location of the purchaser for goods and the ultimate beneficiary for services.
For shareholders, get state of residence on December 31, 2026, any move during the year, and whether each nonresident files other Missouri returns, because that decides the composite and the $1,200 test.
Wording for the client in November: "By December 15, please tell us whether any shareholder moved out of Missouri in 2026 or lives outside Missouri, and send the Missouri tax ID letter. If you want us to evaluate the Missouri pass-through election, we need each owner's other state tax figures by January 15." Those two dates are your own internal deadlines, not Missouri's.
Failure modes and fixes
Extension not claimed on the state form. The federal 7004 was filed, but the Missouri box was not checked. Add the box and the attachment to the review checklist.
Late federal filing carries its own cost. The IRS instructions show $255 per shareholder per month, up to 12 months, plus 5% of unpaid tax per month up to 25%. A $340 penalty may apply for each late or incorrect Schedule K-1. The IRS page does not tie these amounts to a year, so confirm them before quoting on a 2026 return.
Receipts from the wrong state. Services are sourced to where the ultimate beneficiary is, not where the work was done, which can swing the apportionment on MO-MSS.