Kirk went through months of rehabilitation before returning to work in Washington this January. He said in his blog post that he promised himself he would return "with an open mind and greater respect for others."
Kirk is Illinois' ranking Republican lawmaker. His announcement brings to 50 the number of U.S. senators the vast majority of them Democrats who are on record in support of gay marriage, according to Freedom to Marry, a group that supports gay marriage.
It also comes one week after the U.S. Supreme Court held two days of oral arguments on the subject, and as the Illinois Legislature is giving final consideration to a measure that would make Illinois the 10th state in the nation to allow same-sex marriage.
The Illinois Senate voted in February to lift a state ban on same-sex marriage. The legislation also was approved by a House committee, but has yet to be called for a floor vote. Speaker of the House Michael Madigan said recently he believes supporters are a dozen votes short of what they need for the bill to pass.
Gov. Pat Quinn, a Chicago Democrat, has said he would sign the measure.
Kirk's announcement could give political cover to Republicans in the Illinois House who are considering a yes vote but are fearful of a backlash or a primary challenge from social conservatives.
"Sen. Kirk's support should help Illinois House Republicans in particular understand why their support of the measure is the right thing to do," said Bernard Cherkasov, CEO of Equality Illinois.
Sen. Rob Portman of Ohio last month became the first Republican senator to announce his support for gay marriage in states that choose to allow such unions, and GOP Sen. Lisa Murkowski of Alaska says her position is "evolving."
Democratic Sen. Tom Carper of Delaware also announced his support for gay marriage Tuesday, saying on his Facebook page he came to his decision "after a great deal of soul searching." Carper had been one of a dwindling number of Democratic senators who oppose same-sex marriage and are under pressure from liberal groups to change their minds.
Other high-profile Republicans also have recently come out in support of gay marriage, as have a parade of Democrats. The shift reflects growing public support for gay marriage and politicians' efforts to stay in sync with those they represent.
When Gallup first asked in polls about gay marriages, in 1996, just 27 percent felt they should be valid. Now 50 percent of Americans believe same-sex marriages should be recognized by law, with the same rights as traditional marriages, according to Gallup.
Almost two-thirds of Democrats support legalizing gay marriage, compared with 57 percent of independents and 22 percent of Republicans, according to Gallup.
That makes it a political balancing act for Republicans trying to project a more caring and inclusive message without alienating social conservatives who strongly oppose gay marriage and represent some of the GOP's most passionate voters.
Laurie Higgins, a cultural analyst with the Illinois Family Institute, which opposes gay marriage, said Tuesday that she wasn't surprised by Kirk's position because "we've known for a long time he is not a conservative."
Higgins said Portman's announcement made it easier politically for Kirk to express his support, and she called the timing the week before the Illinois House returns to the state Capitol following a mid-session break "suspicious."
"It's much more convenient and comfortable than if he had said this two years ago," Higgins said.
Kirk served five terms in Congress representing Chicago's northern suburbs before he won the 2010 race for President Barack Obama's vacated Senate seat.
He previously voted to end the policy barring gays from openly serving in the military, known as "Don't Ask, Don't Tell," and is a lead co-sponsor of a bill to ban employment discrimination based on sexual orientation.
He also supported Illinois GOP Chairman Pat Brady after Brady announced his support for same-sex marriage earlier this year, drawing the ire of his party's social conservative wing. When some members of the state central committee attempted to oust Brady, Kirk said Brady had his full support.
"We continue to see the momentum behind marriage equality grow, especially among Republicans." said Rick Garcia, Director of the Equal Marriage Illinois Project and Policy Director for The Civil Rights Agenda, Illinois' largest gay rights advocacy organization. "The momentum is stunning and we welcome it."