Senator Mark Kirk has become the latest US lawmaker and second Republican senator to back gay marriage.
In a statement, the Illinois senator said civil marriage should be available to same-sex couples.
Sen Kirk, first elected in 2010, returned to Congress in January after almost a year away following a stroke.
A bevy of US lawmakers have backed gay marriage in recent weeks, reflecting a rapid rise in public support for same-sex marriage.
Last week, the US Supreme Court heard two major cases related to same-sex marriage, one challenging a California ban and the other challenging a federal law restricting tax and other marriage benefits to opposite-sex couples only. The court is expected to issue rulings in June.
Eight Democratic senators have spoken out in support of gay marriage within the last week. In mid-March Ohio Senator Rob Portman became the first Republican senator to support same-sex nuptials, two years after his son told him he was gay.
‘Who you love’
Sen Kirk said that as he climbed the steps of the US Capitol building to return to work in January he promised himself he would “return to the Senate with an open mind and greater respect for others”.
“Same-sex couples should have the right to civil marriage,” he said. “Our time on this Earth is limited, I know that better than most. Life comes down to who you love and who loves you back – government has no place in the middle.”
The senator, who holds the former seat of President Barack Obama, represents a Democratic-leaning state and is considered a moderate Republican.
He previously voted to end the policy barring gay people from serving openly in the US military and is the lead co-sponsor of a bill to ban employment discrimination based on sexual orientation.
His announcement comes as the Illinois state legislature considers a bill that would make the state the 10th to legalise same-sex marriage.
And 50 of the Senate’s 100 members now back civil marriage for same-sex couples, including all but seven Democratic senators, according to a tally by the Reuters news agency. A majority of Republican lawmakers still oppose the move.