CT-1040X: Amended Income Tax Return for Individuals (2025)
Connecticut · Connecticut DRS · Used for tax years 2025
Form CT-1040X is the 2025 amended Connecticut income tax return for individuals. It reports original or previously adjusted amounts, changes, and corrected amounts for income, taxes, payments, and credits, and calculates any refund or amount owed.
Who files this form
Use the 2025 Form CT-1040X to amend a previously filed 2025 Connecticut individual income tax return.
You must file if any of these situations causes your Connecticut income tax to be overpaid or underpaid:
- The IRS or a federal court changes or corrects your federal return.
- You file a timely amended federal return.
- You claimed a credit for taxes paid to a qualifying jurisdiction, and that jurisdiction's tax officials or courts change your return, affecting your allowable credit.
- You claimed that credit and file a timely amended return with the qualifying jurisdiction, affecting your allowable credit.
- You made a mistake or left something out of your Connecticut return.
Do not use this version for another tax year or to amend Form CT-1041 or Form CT-1065/CT-1120SI.
What you'll need
Have your original or previously adjusted Connecticut return available. Attach supporting forms and schedules for the items you change, and write your name and SSN(s) on all attachments.
Depending on your changes, you may also need:
- Federal or other state changes: Copies of the applicable audit results, federal Form 1040X or Form 1045, or the other state's amended return, plus supporting documentation and proof of the final determination.
- Nonresident or part-year resident income: A corrected Schedule CT-SI. Part-year residents must also attach a corrected Schedule CT-1040AW.
- Credit for taxes paid to qualifying jurisdictions: Copies of the returns filed with those jurisdictions.
- Alternative minimum tax or allowable credit changes: Corrected Form CT-6251, Schedule CT-IT Credit, or Form CT-8801, as applicable, marked “Amended.”
- Credits claimed: Schedule 3 for the property tax credit, Schedule CT-EITC, Form CT-1040 CRC, Schedule CT-PE, or the Historic Home Rehabilitation tax credit voucher, as applicable.
Use Forms W-2, 1099, and Schedules CT K-1 to complete the Withholding Schedule when Connecticut tax was withheld, but do not send those documents. If you have more than five, complete and attach Supplemental Schedule CT-1040WH.
How to file
Online: File and pay electronically using myconneCT.
By mail: Send the return to:
Department of Revenue Services
PO Box 2935
Hartford CT 06104-2935
To complete the return:
- Compare your original return with the changes needed. Line numbers may differ from your original return.
- Complete Schedules 1 through 4 with corrected amounts, or the original amounts where nothing changed.
- Enter original or previously adjusted amounts in Column A, net increases or decreases in Column B, and correct amounts in Column C. If a line is unchanged, copy Column A into Column C.
- On Page 3, list each changed line number and explain the change.
- For applicable federal or state changes, check the box on Page 1 and enter the final determination date.
- Sign the declaration on Page 1. Your spouse must also sign a joint return. Keep a copy for your records.
Make a check payable to Commissioner of Revenue Services and write “2025 Form CT-1040X” on it. Adding your SSN(s) to the check is optional. Do not use staples.
Common mistakes
- Using the wrong year's form: The correct year's form must be submitted to avoid processing delays.
- Leaving the return unsigned: Taxpayers must sign the declaration on Page 1.
- Missing credit attachments: The property tax credit requires Schedule 3. The earned income tax credit requires Schedule CT-EITC. The credit for taxes paid to qualifying jurisdictions requires copies of those returns. Missing these attachments will cause the applicable credit to be disallowed.
- Leaving withholding columns incomplete: Complete all columns or your withholding will be disallowed. Do not send Forms W-2, 1099, or Schedules CT K-1.
- Including penalties and interest on Line 23: This line excludes penalties and interest.
- Subtracting out-of-state income on Line 49: The form specifically says not to include it there.
- Using staples: The instructions say not to use them.
Questions people ask
Can I change an overpayment from next year's estimated tax to a refund?
No. Form CT-1040X cannot change that election or your contributions to designated charities.
What if a line has not changed?
Enter the amount from Column A in Column C. For Schedules 1 through 4, use the amounts from your original return where no corrections are needed.
Will I owe interest on additional tax?
Interest accrues at 1% per month or fraction of a month from the original due date until the tax is paid in full. A fraction of a month counts as a whole month.
Can a nonresident or part-year resident claim the Connecticut earned income tax credit?
No. Only full-year Connecticut residents can claim this credit.