Portman, who was on the short list to be 2012 Republican presidential candidate Mitt Romney's running mate, made the announcement in an opinion piece in an Ohio newspaper and gave interviews on his change of heart.
"I have come to believe that if two people are prepared to make a lifetime commitment to love and care for each other in good times and in bad, the government shouldn't deny them the opportunity to get married," Portman wrote in an op-ed piece in the Columbus Dispatch, titled "The Freedom to Marry."
"That isn't how I've always felt. As a Congressman, and more recently as a Senator, I opposed marriage for same-sex couples. Then something happened that led me to think through my position in a much deeper way."
Portman said his 21-year-old son, Will, told the senator and his wife that he was gay in February 2011.
Portman's announcement comes about a week before the U.S. Supreme Court is to hear oral arguments in two cases related to gay marriage. One challenges the 1996 federal Defense of Marriage Act, which defines marriage as a union between a man and a woman.
Portman told the Cleveland Plain Dealer he now believes same-sex couples who marry in states where it is legal should be eligible for the same federal benefits granted to heterosexual couples.
The Republican Party has become increasingly split on the issue, with many arguing socially conservative positions such as opposition to same-sex marriage is contributing to the party's election losses.
Among the headliners expected to speak on Friday is Mitt Romney, who will be making his first major public speech since he lost the presidential election. Portman, whose politically divided home state is key in presidential contests, had a prominent role in Romney's campaign.
In his op-ed piece, Portman wrote of how he has "wrestled" with reconciling his Christian faith with the desire for his son to have same opportunities as his siblings.
"Ultimately, for me, it came down to the Bible's overarching themes of love and compassion and my belief that we are all children of God," he said.
Portman told the Ohio newspapers that because of his son he was able to "think of this issue from a new perspective, and that's of a dad who loves his son a lot and wants him to have the same opportunities that his brother and sister would have - to have a relationship like Jane and I have had for over 26 years."
Portman's son, now a junior at Yale University, told the senator that being gay was "not a choice," and that he had been gay "since he could remember," Portman told the papers.
He added said his views on the matter have evolved over the past two years, and he had consulted clergy members and friends including former Vice President Dick Cheney, who has an openly gay daughter.