Jonathan Alpeyrie, who arrived safely in Paris this week, was kidnapped on April 29 in the town of Yabrud, 75 kilometres (47 miles) north of Damascus, the New-York based agency said.
Alpeyrie had been covering air strikes and shelling of the town by the forces of Syrian President Bashar al-Assad for 10 days before the militia took him to an unknown location, they added.
"He was abducted on April 29 by a Syrian militia," Polaris said in a statement.
"On July 20, after 81 days in captivity, Alpeyrie was released, driven to Lebanon, and he finally arrived in Paris this week".
"His family, friends and colleagues are relieved by the happy ending to this ordeal," added Polaris.
All the photographer's equipment was taken from him by his kidnappers, the agency added.
Polaris did not specify whether Alpeyrie's kidnappers were a rebel contingent or one of Syria's feared groups of Shabiha, armed civilians loyal to Assad acting in parallel to the regular army.
French foreign ministry spokesman Philippe Lalliot confirmed Alpeyrie's release on Friday but refused to give any information on where he was held, who had detained him and how he was freed.
Contacted by AFP, Alpeyrie himself refused to comment.
Born in 1979 in Paris, he moved to the United States in his teens and is now based in New York, according to his website.
Reporters Without Borders describes Syria as "one of the world's most dangerous countries for media personnel", with at least 24 journalists killed and 15 foreign journalists disappearing or being abducted since the start of the conflict in March 2011.
While some kidnappings and detentions are made public, others are kept under wraps by the family, employer and by governments in a bid to facilitate any potential release - as was the case for Alpeyrie.
Lalliot said it is France's policy to refuse any comment about hostage cases even after their release.