Judges on Connecticut's highest court voted four to three that the state's constitution does not permit barring gay and lesbian couples the same rights accorded to heterosexuals.
The ruling follows similar decisions by Massachusetts, also in the northeast, and the west coast state of California.
"In accordance with these state constitutional requirements, same sex couples cannot be denied the freedom to marry," said Judge Richard Palmer, speaking for the majority opinion.
"Even though the right to marry is not enumerated in our constitution, it long has been deemed a basic civil right," Palmer said.
The ruling came in response to a 2004 lawsuit challenging the ban on gay marriage.