French academic Clotilde Reiss, who was charged with spying, was questioned before a judge on Tuesday but then allowed to return to the French Embassy, rather than prison, a French official said.
The mass trial of opposition figures and activists began in August, with Iran's rulers accusing them of plotting what they have depicted as a foreign-backed plot to oust them from power.
In the weeks following the June 12 election, the opposition led massive street protests over what it saw as fraud after election authorities declared Mahmoud Ahmadinejad the winner of a second presidential term.
Rallies drew hundreds of thousands of people and clashes with security forces, marking the most serious internal unrest in Iran since the Islamic Revolution 30 years ago.
The opposition says more than 70 people were killed in the unrest following the polls while the government puts the number of dead at about half that figure.
Despite the crackdown, the government has stopped short of indicting the most prominent opposition leaders, such as presidential candidates Mir Hossein Mousavi and Mahdi Karroubi.
The opposition continues to claim that Mousavi was the rightful winner of the election.
In October, the Iranian news agency ISNA reported that three people had been sentenced to death on charges related to post-election unrest.
They were also accused of affiliation to a pro-monarchy group and the People's Mujahideen Organisation of Iran (PMOI), which is an exiled opposition group, seen by both Iran and the US as a terrorist group.
Last month, Navi Pillay, the UN High Commissioner for Human Rights, urged the country's appeal courts to review the three death sentences.