The man, named as Najim Laachraoui, 24, is still on the run. A statement said he had been using false ID and that his DNA had been found in houses used by the suspected jihadist network.
Abdeslam was captured in Brussels on Friday and is still being interrogated.
However, Belgium's federal prosecutor said he was "still far from solving the puzzle" of the Paris attacks.
"We have quite a few pieces of the puzzle and in the last few days several pieces have found their place," Frederic Van Leeuw told journalists in Brussels.
So-called Islamic State (IS) said it was behind the Paris attacks.
In other developments:
Salah Abdeslam's luck runs out
Is Molenbeek a haven for Belgian jihadis?
What happened during the Paris attacks?
Who were the Paris attackers?
The prosecutor's statement said that Abdeslam had travelled twice to the Hungarian capital Budapest, using a rental car last September.
In the car were two other men, using fake Belgian identity cards with the names Samir Bouzid and Soufiane Kayal.
Soufiane Kayal has now been identified as Laachraoui by DNA found at houses in the town of Auvelais and the Brussels district of Schaerbeek.
"The investigation showed that Soufiane Kayal can be identified as Najim Laachraoui, born on 18 May 1991 and who travelled to Syria in February 2013," the statement said.
Belgian police said Samir Bouzid was "most probably" Mohamed Belkaid. He was killed by a police sniper in a raid on a flat outside Brussels on 15 March.
The prosecutor appealed for public help in finding Laachraoui.
Also still sought is Mohamed Abrini, who was filmed at petrol stations with Abdeslam two days before the Paris attacks.
Abdeslam's lawyer, Sven Mary, meanwhile told Belgian radio that his client appeared "relieved" that the hunt was over.
Mr Mary said: "Salah wants to co-operate and he's done that since Saturday. Are we going to apply for the status [of informer]? He's never said he wants to be an informer.  I've never said that he's asked for that."
But Mr Mary added: "I have said that he can be of invaluable worth for different parties. He can give invaluable information for police as well as judges and lawyers.
"The statements of the French prosecutor... could ensure that Abdeslam is scared off."
Mr Mary was referring to comments over the weekend by Paris prosecutor Francois Molins that Abdeslam had admitted he wanted to blow himself up during the attacks, but then changed his mind.
Mr Mary has filed a legal complaint against the prosecutor for violating secrecy.
Mr Mary also said he would continue to fight a transfer to France, but added: "Let's be quite clear. He's going to France - there is no single reason that he won't go to France. It'll be the  investigating judge who decides when he goes."
The BBC's Lucy Williamson in Paris says that for those who lost relatives in the attacks, relief at Salah Abdeslam's arrest is quickly turning to hunger for the information he might provide.
"We hope [his arrest] will help us in the search for truth" read the message from one support group set up after the November attacks.
But our correspondent says that Mr Hollande's first formal meeting with relatives comes amid recrimination that they were ignored for four months.
Earlier this month, Georges Salines, who lost a daughter in the Bataclan theatre during the attack, said he had written to Francois Hollande to ask for such a meeting, but was "stunned" to be told that there was no time in the president's schedule.
France has tightened security at its borders after the arrest of the man dubbed Europe's most-wanted fugitive.
Mr Hollande has said that the number of people involved in the terrorist network is much larger than originally thought and that he wants Abdeslam transferred to France as soon as possible to face prosecution.
Belgian Foreign Minister Didier Reynders has suggested Abdeslam was preparing attacks in Brussels before he was arrested.
The Belgian authorities have charged Abdeslam with terrorism offences.
The 26-year-old French national, born in Belgium, spent four months on the run.
Another man arrested at the same time as Salah Abdeslam, Monir Ahmed Alaaj, has also been charged with participation in terrorist murder and the activities of a terrorist group, Belgian prosecutors say.
Belgian prosecutors say that DNA has identified an accomplice of captured Paris attacks suspect Salah Abdeslam.