**The information below has not been verified for the 2020 tax year as the Connecticut Form CT-1040 instructions have not yet been released by the state.**
Per the Connecticut Form CT-1040 Resident Income Tax Return Instructions, on page 29:
If you are a resident of Connecticut and if any part of your income was taxed by a qualifying jurisdiction, you may be able to claim a credit against your Connecticut income tax liability for qualifying income tax payments you have made.
A qualifying jurisdiction includes another state of the United States, a local government within another state, or the District of Columbia. A qualifying jurisdiction does not include the State of Connecticut, the United States, or a foreign country or its provinces (for example, Canada and Canadian provinces).
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.