The announcement was made by Mohammad Shadabi, an official within the Tehran prosecutor's office.
Haleh Esfandiari's lawyer is Nobel Peace Prize winner Shirin Ebadi. She told Radio Farda that the order for Esfandiari's release was signed today after bail of 3 billion rials ($320,000) was posted.
"Esfandiari was released on bail," Ebadi said. "I'm really happy that 3 1/2 months after her illegal arrest, the judiciary finally decided to respect the law and release her."
Ebadi said her client, who heads the Middle East Program at the Woodrow Wilson International Center for Scholars in Washington, was held in solitary confinement with no personal contact with their families or lawyers.
"I really regret that they held a 68-year-old lady, who is not in very good health, for 3 1/2 months in solitary confinement, without any acceptable reason," Ebadi said.
Ebadi also told Radio Farda that she is optimistic that Esfandiari will be acquitted when her case goes to court.
"Right now, from the judiciary's point of view, the case is open," Ebadi said. "But [Haleh Esfandiari] will be out of prison until the day of the court hearing, and I'm sure that she will be acquitted because I'm completely [convinced] of her innocence. The three months she spent in solitary confinement was against the law."
Iran's judiciary announced on August 12 that they had completed their investigations into Esfandiari's case and that of another detained Iranian-American -- a consultant with the Open Society Institute, Kian Tajbakhsh.
Both appeared on Iranian state television in July in a program that Iranian officials claimed proved that the two are linked to a U.S. plot to destabilize Iran's Islamic establishment.
Both have been in Tehran's notorious Evin prison since their arrests in May.
There was no word on Tajbakhsh or the fate of another detained U.S.-Iranian, peace activist Ali Shakeri.
Authorities have also refused to allow a fourth Iranian-American, Radio Farda broadcaster Parnaz Azima, to leave Iran. They have charged Azima with working for Radio Farda and spreading propaganda against the Iranian state.
Human rights groups and a number of U.S. politicians have called on Iran to released all of the detained U.S.-Iranians.
(Radio Farda, AP)