According to AFP and Le Parisien, Foued Mohamed Aggad went to Syria with his brother and a group of friends at the end of 2013.
The source said most of the others were arrested in spring last year when they returned to France, however Aggad stayed in Syria.
The attacks on the Bataclan - during an Eagles of Death Metal concert on 13 November - left 90 dead.
The three gunmen wearing suicide belts burst into the venue and opened fire.
The band members escaped the carnage by hiding in a dressing room, but their merchandising manager, Nick Alexander, 36, from Colchester, Essex, was among the victims.
Frontman Jesse Hughes said: "A great reason why so many were killed was because so many people wouldn't leave their friends, and so many people put themselves in front of people."
The attackers all died at the scene. Two had already been identified as Omar Omar Ismail Mostefai, 29, and 28-year-old Samy Amimour.
Mostefai was identified using a fingertip found at the venue.
Police say Aggad was identified at the end of last week after his DNA was matched with those of his family members.
