In a statement posted on his website, Kirk said, "Life comes down to who you love."
Kirk said he returned to Congress after his stroke with an open mind. Kirk is now the 50th senator to support same-sex marriage.
While Senator Kirk's same-sex marriage switcheroo is significant, it's not entirely unexpected. After all, it was Kirk who helped protect the Illinois Republican party chair Pat Brady from conservatives in the party who were trying to oust Brady for also supporting same-sex marriage.
The stroke changed his life and -- on some issues -- his mind. So says Senator Mark Kirk, who posted a simple four-sentence message Tuesday, announcing he now supports same-sex marriage.
"Same-sex couples should have the right to civil marriage" Kirk wrote. "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."
"Most of us have gay acquaintances at work or church," said Kirk in an interview with the Illinois Radio Network. "And we know them and the thought of legally discriminating against our own friends and co-workers is an anathema to me."
Kirk becomes only the second sitting Republican Senator to come out in support of gay marriage. He joins Ohio Senator Rob Portman, who announced his change of heart after his son revealed to the family he was gay.
"Mark has been a good vote getter. He's a good Republican but certainly he has been more liberal than the base of the Republican Party," State Senator Jim Oberweis, (R) Sugar Grove, said.
In January, Kirk talked with ABC7's Paul Meincke about how his stroke changed his outlook.
"The thought of going to work with them to overcome some of the partisan rancor in the Senate, that is so evident," Kirk said in January.
But Kirk says marriage laws should be set by the states, not a decree from Congress.
"I would hope we would restrain our appetite for power in Washington and not take over marriage law for the whole country," Kirk said Tuesday.
"It's not totally surprising because Mark Kirk has always been moderate, and he's one of the main sponsors of the federal gay rights bill," said Equal Marriage Illinois Project's Rick Garcia.
Garcia said Kirk's support may improve the chances of an Illinois same-sex marriage bill picking up support from more downstate lawmakers. However, some lawmakers say gay marriage is not a bipartisan issue.
"It really provides cover, especially in Illinois," Garcia said, "for those members of the Illinois House of Representatives looking at a marriage bill, and this provides cover for them."
"This is not really a partisan issue. We have hundreds of thousands of Democrats across the state who support traditional marriage," Paul Caprio, Coalition to Protect Children and Marriage, said.
As for a gay marriage bill in the General Assembly, supporters will try again later this spring.