It said Saeed Shariati, one of opposition leader Mir Hossein Mousavi's campaign aides during the disputed June election, has been released on the order of Tehran's Revolutionary Court.
The agency gave no further details, but reported separately that a group of 12 people who had been detained in two protest gatherings since last month have also been released after questioning.
Shariati was among more than 100 opposition politicians and activists who have been on trial since August on charges of fomenting the street unrest that broke out after President Mahmoud Ahmadinejad's disputed re-election in June.
In the days after the June 12 election, hundreds of thousands of opposition supporters poured into the streets of major Iranian cities to protest what they said was a fraudulent result.
The opposition says at least 72 people were killed in the ensuing security crackdown on protesters and that many of those detained were abused in custody. The government puts the number of dead at 30.
Mousavi claims he was the rightful winner of the election and that Ahmadinejad was declared the winner after massive vote fraud.