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.
Bak Sahraei, the second counselor of Iran's UN mission, sent an e-mail confirming the release of Shourd, following up an earlier text message from the Culture Ministry telling reporters them to come to a Tehran hotel on Saturday morning to witness the release.
The site is the same one where the three were allowed the only meeting with their mothers since they were detained in July 2009.
Iran claims they illegally crossed the border from Iraq's northern Kurdish region and had threatened to put the three on trial for spying. Their families say they were hiking in the largely peaceful region of Iraq and that if they crossed the border, it was accidental.
"Offering congratulations on Eid al-Fitr," the ministry text message said, referring to the feast that marks the end of Ramadan.
"The release of one of the detained Americans will be Saturday at 9 a.m. at the Estaghlal hotel."
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.
Iranian President Mahmoud Ahmadinejad has in the past proposed swapping the three for Iranians he says are jailed in the U.S., raising fears that the Americans are being held as bargaining chips.
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.
Releasing prisoners and showing clemency is a common practice in the Muslim world during the fasting month of Ramadan. Iran's official IRNA news agency said Supreme Leader Ayatollah Ali Khamenei has already pardoned a group of prisoners for Eid al-Fitr. The report gave no number of the freed inmates and did not say whether they also included the American.
Shourd, 31, had told her mother she has serious medical problems.
Nora Shourd, said her daughter told her in a telephone call in August that prison officials have denied her requests for medical treatment. The mother said they talked about her daughter's medical problems, including a breast lump and precancerous cervical cells, and her solitary confinement in Tehran's Evin prison.
During the American hostage crisis in 1979-1981, Iran first released women and African-Americans as a sign of respect for women and mercy toward minorities.
In Washington, State Department spokesman Mark Toner said U.S. officials are in contact with Swiss diplomats who handle U.S. affairs in Iran.
"We don't know, frankly, what Iran is contemplating at this point," Toner said. "If this turns out to be true, this is terrific news. The hikers' release is long overdue."
A statement by Samantha Topping, a New York-based spokeswoman for the three mothers, said they are "urgently seeking further information."
"We hope and pray that the reports are true and that this signals the end of all three of our children's long and difficult detention," the statement said. "Shane, Sarah and Josh are all innocent and we continue to call for their immediate release, so that they can return home together and be reunited with our families."
Bauer's mother, Cindy Hickey of Pine City, Minnesota, told The Associated Press that the mothers had hoped for a release during Ramadan because they knew it was a tradition. She said she was excited about the release, even if the hiker being freed isn't her son.
"I'm hoping that even if one is released, the other two will follow," Hickey said. "I'm holding my breath for the official word on this."
Iranian leaders have repeatedly suggested a link between the jailing of the Americans and Iranians they claim are held by the United States.
The Swiss embassy in Tehran has handled consular affairs for the United States for about 30 years, since after 1979 Iranian revolution. Swiss diplomats refused to comment Thursday on any possible release of the three detained Americans but are expected to be involved in any transfer.
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.
Ali Reza Shiravi, the head of Iran's foreign media office at the Culture Ministry, confirmed that he had sent the message summoning reporters to the hotel.
The high-rise Estaghlal hotel near Evin prison is where the three Americans' mothers were allowed to visit them in May in a highly publicized trip.
Iran's president has in the past suggested the Americans could be traded for Iranians claimed to be held by the U.S.
Iran in December released a list of 11 Iranians it says are being held in the U.S. They included a nuclear scientist, Shahram Amiri, who disappeared in Saudi Arabia but has since returned to Iran.
Also on the list were a former Defense Ministry official who vanished in Turkey and an Iranian arrested in Canada on charges of trying to obtain nuclear technology. Three of those on the list have been convicted or charged in public court proceedings in the United States. The circumstances surrounding some of the others are more mysterious.
Ali Reza Asgari, a retired general in the elite Revolutionary Guard and a former deputy defense minister, disappeared while on a private trip to Turkey in December 2006.
The list also includes three Iranians who Tehran claims were abducted in Europe and sent to the U.S.: merchant Mohsen Afrasiabi, who it says disappeared in Germany, as well as electrical engineering student Majid Kakavand and a former ambassador to Jordan, Nasrollah Tajik, who it says vanished in France.
Associated Press Writers Edie Lederer at the United Nations, Patrick Condon in Minneapolis, Jeff Baenen in Pine City, Minnesota, and Matthew Lee in Washington contributed to this report.