Sen. Mark Kirk of Illinois, a socially moderate Republican, announced on Tuesday his support for same-sex marriage, becoming the second Republican in the chamber to support it.
"Same-sex couples should have the right to civil marriage," he wrote in a statement. "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."
Sen. Rob Portman of Ohio became the first Republican in the U.S. Senate to endorse same-sex marriage. He announced last month that he was swayed on the issue by his son, who told his family he was gay.
Seven of the 55 Democratic senators have not endorsed same-sex marriage. Sen. Tom Carper, D-Delaware, announced his support earlier on Tuesday.
The Supreme Court heard oral arguments last week in two cases on the issue, including a challenge to the federal Defense of Marriage Act that defines marriage as between a man and a woman.
Kirk, 53, returned to the Senate in January after spending a year in recovery from a stroke.
"When I climbed the Capitol steps in January, I promised myself that I would return to the Senate with an open mind and greater respect for others," he said in Tuesday's statement.
He served in the U.S. House from 2000 to 2010, where he voted twice against constitutional amendments banning same-sex marriage.
Kirk holds the Senate seat which President Barack Obama vacated in 2008.