Her British husband Richard Ratcliffe said Ms Zaghari-Ratcliffe, a British-Iranian citizen, was with family in Damavand and had been reunited with her daughter Gabriella, four.
He said: “The Free Nazanin campaign is very pleased to confirm that Nazanin was released from Evin prison on furlough this morning.
“Initially the release is for three days - her lawyer is hopeful this can be extended. She is currently with her family in Damavand.
“This was a very happy surprise after a number of false dawns recently, which had been increasingly unsettling.”
Describing the family reunion in Damavand, just outside the capital Tehran, Mr Ratcliffe said his wife was “overwhelmed”.
He said: “Nazanin travelled to Damavand from Evin with her father and brother to rejoin the family holiday, and see Gabriella, her grandmother and the wider family.
“Gabriella had picked some flowers from the family garden, and was waiting to present them - the tradition she has seen for families waiting outside Evin prison. Gabriella has promised to show her the garden, and also her dolls house.
“She (Nazanin) was very happy on the phone, though confessed to having cried lots, particularly when seeing her grandmother, and to being still overwhelmed. “
Mr Ratclifffe also thanked Foreign Secretary Jeremy Hunt.
He said: “Our thanks to all those involved in making this possible in Tehran and London, and to the new Foreign Secretary for all his recent efforts and considerations.”