Kirk, 53, recently returned to Congress after suffering a stroke last year. During his recovery, Kirk says he promised himself he would return to the Senate with an open mind and greater respect for others.
His statement reads in part, “Same-sex couples should have the right to civil marriage. 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 was 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.
Six of the 55 Democratic senators have not endorsed same-sex marriage. Sen. Tom Carper, D-Delaware, announced his support earlier on Tuesday.
Kirk holds the Senate seat which President Barack Obama vacated in 2008.
The CNN Wire Service contributed to this report.