The Ohio Republican informed reporters from several newspapers in his home state of his reversal, which The Columbus Dispatch calls "stunning."
Portman told The Cincinnati Enquirer his evolution on the subject began in 2011 when his son, Will, then a freshman at Yale University, told his parents he was gay.
Portman told the Enquirer his new views reflect "a change of heart from the position of a father" and that he first talked to his pastor and others, including former Vice President Dick Cheney, whose daughter is a lesbian.
As a member of the House in 1996, Portman co-sponsored the Defense of Marriage Act (DOMA), which defines marriage as between a man and a woman and bars federal recognition of same-sex marriage.
The Supreme Court is slated to hear arguments on a challenge to DOMA next week.
"Like former President Bill Clinton, who signed the law," the Dispatch says, "Portman now wants the high court to invalidate the law's declaration that marriage is between a man and a woman. Instead, Portman said he would prefer that it be left to the states to decide the definition of marriage."
In an interview with CNN, Portman explains, "I've come to the conclusion that for me, personally, I think this is something that we should allow people to do, to get married, and to have the joy and stability of marriage that I've had for over 26 years. That I want all of my children to have, including our son, who is gay.""One way to look at it," Portman wrote in an OpEd in Friday's Dispatch, "is that gay couples' desire to marry doesn't amount to a threat but rather a tribute to marriage, and a potential source of renewed strength for the institution."