The plaintiffs in the case said they were delighted with the ruling
Connecticut's Supreme Court has overturned a ban on same-sex marriages, making it the third US state to legalise such unions.
The court ruled that the law limiting marriage to heterosexual couples was unconstitutional as it discriminated on the basis of sexual orientation.
Gay marriage was legalised in California earlier this year and in Massachusetts in 2003.
Connecticut already permits same-sex civil unions.
The Connecticut Supreme Court ruled 4-3 that a ban on gay marriage infringed on a "fundamental right" of same-sex couples.
The ruling followed four years of legal wrangling in the state court system.
The plaintiffs in the case - eight same-sex couples - said they were delighted by the outcome.
"We are overjoyed to tell our twin boys that we will be married, just like their friends' parents," partners Elizabeth Kerrigan and Jody Mock said in a statement.
The governor of Connecticut, Jodi Rell, a Republican, said she did not agree with the ruling but would uphold it.
"I continue to believe that marriage is the union of a man and a woman. I do not believe their [the judges'] voice reflects the majority of the people of Connecticut," she said.
The governor said she believed any attempt to contest the decision, either legislatively or by amending the state constitution, would fail.