To file a Missouri amended tax return, use Form MO-1040 for the tax year you are correcting and check the “amended return” box at the top of the form.1Missouri Department of Revenue. How Do I Correct a Filing Mistake Missouri has no separate amendment form. You complete the entire return using your corrected numbers and attach documentation showing what changed. To claim a refund, you generally have three years from the date you filed the original return or two years from the date you paid the tax, whichever is later.2Missouri Revisor of Statutes. Missouri Code 143.801 – Limitations on Credit or Refund
When You Need to Amend
Common triggers are a corrected W-2 or 1099, a change in filing status, a deduction or credit you missed, or income you later discovered you failed to report. If the underlying mistake was on your federal return, file federal Form 1040-X with the IRS first. Your completed federal amendment becomes a required attachment to the Missouri filing.3Missouri Department of Revenue. Form MO-1040 Individual Income Tax Long Form and Instructions
The 90-Day Rule for Federal Changes
If the IRS audits your federal return and changes your taxable income, or you file an amended federal return on your own, Missouri law gives you 90 days from the final federal determination to file an amended state return.4Missouri Revisor of Statutes. Missouri Code 143.601 – Report of Change or Correction of Federal Taxable Income The amended report must also state whether you believe the federal determination is wrong and explain why.
Missing that window costs you. If the federal change would have produced a state refund, interest on the refund stops accruing after day 90.2Missouri Revisor of Statutes. Missouri Code 143.801 – Limitations on Credit or Refund If you owe more state tax because of the federal change, you also give up the ability to argue that the assessment period has closed.
Deadline to Claim a Refund
For an amendment unrelated to a federal change, the refund window is three years from the date you filed the original return or two years from the date you paid the tax, whichever is later.2Missouri Revisor of Statutes. Missouri Code 143.801 – Limitations on Credit or Refund If you never filed for that year, the deadline is two years from the date the tax was paid. File one day late and the Department of Revenue will deny the refund regardless of the merits.
Filling Out the Amended MO-1040
Download the MO-1040 and instructions for the specific tax year you are correcting from the Missouri Department of Revenue website. Do not use the current year’s form to fix a prior year. Each tax year has its own version.1Missouri Department of Revenue. How Do I Correct a Filing Mistake
Check the “amended return” box at the top. Then complete the entire five-page MO-1040 and the three-page Individual Income Tax Adjustments form (MO-A) using your corrected figures, not the numbers from your original return.3Missouri Department of Revenue. Form MO-1040 Individual Income Tax Long Form and Instructions You are rewriting the return as if you were doing it correctly the first time, not just listing the changes.
Reason for Amending
The form asks you to check a box explaining why you are amending. The choice controls what supporting documents the Department of Revenue expects.3Missouri Department of Revenue. Form MO-1040 Individual Income Tax Long Form and Instructions
- Box A, federal audit. You received an IRS notice or know your original federal return was incorrect. Attach a copy of your amended federal return or the IRS revenue agent’s report, and enter the date the audit was finalized.
- Box B, net operating loss carryback. Enter the year the loss occurred.
- Box C, investment tax credit carryback. Enter the year the credit arose.
- Box D, amended federal return or Missouri correction. You filed an amended federal return or are fixing a Missouri-specific error. Enter the date you filed the amended federal return.
Most people check Box A or Box D. If you are correcting a Missouri-only mistake, such as a property tax credit you forgot to claim, Box D is the right choice.
What to Attach
An amended return without proper attachments sits in a queue while the Department of Revenue contacts you for the missing pieces. Include everything the first time.
For every amended return, attach all supporting schedules, a copy of your federal changes, and your federal Form 1040-X.3Missouri Department of Revenue. Form MO-1040 Individual Income Tax Long Form and Instructions If the change came from an IRS audit rather than your own amendment, attach the IRS revenue agent’s report in place of Form 1040-X. Add any corrected income documents involved, such as W-2s, 1099s, or K-1s, and any Missouri-specific forms you adjusted, like the MO-A.
If you need to prove what the IRS actually changed on your federal account, request a tax account transcript from the IRS. That transcript type shows changes made after your original federal return was filed.5Internal Revenue Service. Transcript Types for Individuals and Ways to Order Them
Submitting the Return and Paying What You Owe
Amended returns are submitted by mail. The Missouri Department of Revenue uses different addresses for refund returns and balance-due returns, so check the MO-1040 instructions for the tax year you are filing to confirm the correct one. Using the wrong address slows processing.
If the amended return shows tax due, send payment with the return. You can pay by check or money order made out to the Missouri Department of Revenue. Missouri also accepts electronic bank drafts (e-check) through its online payment portal for a $0.50 handling fee, and credit or debit cards online for a higher processing fee.6Missouri Department of Revenue. Pay Individual Income Taxes Online
Interest and Penalties on Additional Tax
When the amendment shows you underpaid, interest runs from the original due date of the return, not from the date you file the amendment. Missouri sets the rate under Section 32.065 and adjusts it periodically. The underpayment rate was 9% during 2024 and 8% for 2025.7Missouri Revisor of Statutes. Missouri Code 143.731 – Interest on Underpayment No interest is charged if the amount due is less than one dollar.
You may also owe an underpayment penalty if the shortfall for the tax year exceeds $500. Missouri calculates it on Form MO-2210, which prorates the charge across quarterly estimated-payment periods. Below the $500 threshold, the penalty is waived.
If you find you owe more, amend quickly. Every day between the original due date and payment adds interest.
Checking the Status of Your Amended Return
Missouri’s Return Tracker at mytax.mo.gov shows whether your return has been received and whether it was processed as filed or adjusted.8Missouri Department of Revenue. Obtaining the Status of Your Tax Return You will need your Social Security number, filing status, and the whole-dollar refund or balance-due amount from the return.9Missouri Department of Revenue. Missouri Return Tracker
For live help, call the Department of Revenue at (573) 751-3505 or email income@dor.mo.gov.10Missouri Department of Revenue. How Do I Check on the Status of My Personal Income Tax Return Amended returns go through manual review and take considerably longer than original electronically filed returns. If the Department needs more information, it sends a written notice, so watch your mail for several months after filing.