"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 wrote.
The well-known Ohio conservative, a former U.S. trade representative and White House budget chief, was considered but not chosen as Republican presidential nominee Mitt Romney's running mate. Portman told the newspapers Romney was informed about Will's sexuality last year.
Portman's reversal comes as the U.S. Supreme Court is expected to hear arguments this month in a challenge to a provision of the Defense of Marriage Act.
Portman said he would like Congress to repeal the provision of the DOMA that bans federal recognition of gay marriage, though he still supports the part of the law that says states should not be forced to recognize such marriages.
A group working to overturn Ohio's ban on same-sex marriage praised Portman's comments, as did Ohio's senior senator, Democrat Sherrod Brown.
"I look forward to working with him to ensure that all Americans have the ability to marry regardless of whom they love or where they live," Brown said in a written statement.
Brown voted against DOMA while he was a member of the U.S. House.