Sarah Shourd and two friends were arrested along the Iran-Iraq border in July 2009, and Iran has accused them of illegal border crossing and spying. Their families say they were hiking in Iraq's scenic north and that if they crossed the border, they did so unwittingly.
There was no word on the fate of the other two Americans, Josh Fattal, 28 and Shane Bauer, 28, to whom she got engaged to while they were in prison.
The gesture could be a calculated move by Iran to soften international criticism of its judiciary. Iran has faced a growing storm of protest over a stoning sentence for a woman convicted of adultery that has been temporarily suspended.
Iran's Mehr news agency on Friday quoted the Foreign Ministry as saying the president intervened in part because of the "special viewpoint of the Islamic Republic of Iran on the dignity of women."
The 31-year-old Shourd has been held in solitary confinement, and her mother says she's been denied treatment for serious health problems.
Once the American is released, normal protocol would be to turn the person over to Swiss diplomats to be taken to the embassy.
There are direct commercial flights to Geneva a few times a week. While flights to Dubai, such as the one taken by the Americans' mothers, are much more frequent, they are probably all booked because of the holidays.
If the released American requires medical care, Geneva would also be the more attractive option.
The imprisonment of the Americans has deepened tensions between the U.S. and Iran, a relationship already strained over Washington's suspicions that Tehran is trying to manufacture nuclear weapons -- something Iran denies.