The suspected accomplice was named as Najim Laachraoui, who was previously known by the false name Soufiane Kayal which he used to travel to Hungary in September with Salah Abdeslam, the key suspect who was arrested in a dramatic raid on Friday.
Traces of the genetic material of 24-year-old Laachraoui, who left for Syria in 2013 and is still on the run, were found on the bombs used in the November 13 gun and suicide bomb attacks in Paris, a source close to the French investigation said.
French President Francois Hollande, who has said he wants Abdeslam extradited as quickly as possible, was due to hold his first formal meeting with relatives of the 130 Paris victims this afternoon.
"The president, in light of recent events, will update them on what is happening," the presidential palace said.
Investigators hope Abdeslam's arrest in Brussels on Friday, in which he was wounded in the leg, will give new leads on the attacks claimed by the Islamic State jihadist group.
Abdeslam, the last known survivor of the group that carried out the attacks, was found just around the corner from his family home in the gritty Brussels neighbourhood of Molenbeek, where several of the Paris attackers hailed from.
Belgium has faced heavy criticism for failing to keep tabs on Islamic radicals there.
The investigation is now widening, and Hollande has said that the network involved in the Paris attacks was much bigger than previously thought.
French prosecutor Francois Molins will meet his Belgian counterpart Frederic Van Leeuw and hold a press conference, Belgium's federal prosecutors' office said.
Molins said at the weekend that Abdeslam played a "central role" in the attacks and originally planned to "blow himself up" at the Stade de France stadium but changed his mind.
"These first statements, which should be taken cautiously, leave a whole series of issues that Salah Abdeslam must explain," he added.
Belgian Foreign Minister Didier Reynders said on Sunday that Abdeslam -- who has been charged with "terrorist murder" and belonging to a terrorist group -- had already told investigators he was planning some sort of new attack in Brussels.