"I think we should be allowing gay couples the joy and stability of marriage," Portman told The Cincinnati Enquirer Thursday.
While serving in the House in 1996, Portman supported the Defense of Marriage Act, which defined marriage as between a man and woman, The Hill noted.
In an op-ed piece he wrote for The Columbus Dispatch, Portman said his son's disclosure to him and his wife Jane caused him to re-evaluate his stance.
"Jane and I were both surprised, very surprised, but also very supportive of him. Our reaction was not about policy or positions. It was about him as a son and letting him know we were 110 percent supportive of him," Portman said.
His son's homosexuality "allowed me to think about this issue from a new perspective, and that's as a dad who loves his son a lot," he said.
Portman said he wants his son to have the opportunity for a long-lasting relationship, "like Jane and I have had for over 26 years."
Portman -- the only current Republican U.S. senator to back same-sex marriage -- said he doesn't plan to take a leadership role on the issue but wanted to inform his constituents of his shift in position, The Hill reported.
White House spokesman Josh Earnest told reporters Friday Portman's comments reflect "a pretty significant sociological shift in this country that's happening at a pretty rapid pace, and it's happening right before our eyes in a way that says a lot about our country -- that we have a country where we prioritize equality and fairness."
Earnest said the change is also evident in the "large number of Republicans" who have signed onto briefs urging the U.S. Supreme Court to uphold a lower court ruling that Proposition 8 -- a voter approved referendum that strips same-sex couples of the right to marry in California -- violates the U.S. Constitution.