Tehran prosecutor Saeed Mortazavi said Reiss was released after six weeks in the capital's Evin prison on Sunday after posting bail of 300,000 dollars (213,000 euros), the Mehr news agency reported.
Mortazavi had said earlier that the hearing in her case had ended, but that she could not leave the country before the court reached its verdict, without specifying when the ruling was expected.
Reiss, 24, was arrested at a Tehran airport on July 1 for taking part in mass protests triggered by President Mahmoud Ahmadinejad's re-election the previous month.
When she appeared in court in a mass trial on August 8, Iranian state media said she was accused of "collecting information and provoking rioters."
Reiss had planned to fly home after completing a six-month teaching and research assignment in the central city of Isfahan. In the closing weeks of her stay she witnessed the protests, took pictures and emailed them to friends.
France has insisted that Reiss is not guilty and has demanded her unconditional release.
Iran has blamed Western governments, especially the United States and Britain, for the riots and accused opposition supporters of seeking to topple the Islamic regime through a "soft coup" with foreign help.
The opposition dismisses such allegations, insisting that the June 12 re-election of Ahmadinejad was massively rigged to keep the hardliner in power.
Scores of reformists, journalists and opposition supporters were jailed in the aftermath of the election and around 140 have appeared in the dock in mass trials.
