"Our time on this Earth is limited, I know that better than most," said Kirk, who suffered a stroke in January 2012. "Life comes down to who you love and who loves you back — government has no place in the middle."
Kirk went through months of rehabilitation before returning to work in Washington this January. He said in his blog post that when he went back to the Senate he promised himself he would return "with an open mind and greater respect for others."
Kirk is Illinois' ranking Republican lawmaker. His announcement comes less than three weeks after Sen. Rob Portman of Ohio became the first Republican in the Senate to say he supported gay marriage.
Kirk previously voted to end the policy barring gays from openly serving in the military, known as "Don't Ask, Don't Tell." He's a lead co-sponsor of a bill to ban employment discrimination based on sexual orientation, and has opposed a constitutional amendment to ban gay marriage.
He also supported Illinois GOP Chairman Pat Brady after Brady announced his support for same-sex marriage earlier this year. When some members of the state central committee attempted to oust Brady, Kirk said Brady had his full support.