Sporting a trimmed beard and wearing a navy blue sweater, Nemmouche showed no emotion and stared into space as the verdict was delivered Thursday evening.
The 12 jurors, accompanied by the presiding judge and two other magistrates, had deliberated for two and a half days in a closed session at a Brussels hotel before returning their verdict.
He now faces a life sentence for the anti-Semitic gun attack following his return from Syria's battlefields.
"Trick" claims dismissed as lacking credibility
33-year-old Nemmouche was found to have killed the four victims in cold blood in less than 90 seconds, in the Belgian capital on May 24th 2014, but he denied the accusation telling the court he had been "tricked".
Presiding judge Laurence Massart countered that "The existence of a trap was not presented with enough credibility and must be ruled out."
Nemmouche's lawyers had argued he was not to blame for the slaughter, but had been caught up in a plot targeting the Israeli intelligence agency Mossad,
The legal argument had centred around Israeli couple Miriam and Emmanuel Riva, the first two of the four people shot dead in the attack, alledging they were in fact Mossad agents murdered by another man who had hunted them down.
A young Belgian employee, Alexandre Strens, and French volunteer Dominique Sabrier were also murdered.
The Riva family's lawyers rejected the theory and said attempts to pass off the tourists as secret agents was "an absolute scandal".
Miriam Riva worked for Mossad but, as an accountant, she was not operational, according to the investigating judges who travelled to Israel during their inquiry.
Accomplice Nacer Bendrer also found guilty
The 12 jurors also found fellow Frenchman Nacer Bendrer, who was accused of supplying the weapons, to be the co-author of the attack.
Seated next to Nemmouche in the defendant's box, encased by bullet-proof glass on the sides, Bendrer then hung his head low for a few minutes before covering it with his hands.
He also faces a life jail sentence.
The investigation showed that the two men had dozens of telephone conversations in April 2014, when Nemmouche was preparing for the killings.
Six days after the massacre, Nemmouche was arrested in the southern French city of Marseille in possession of a revolver and a Kalashnikov-type assault rifle.
Bendrer admitted that Nemmouche had asked him for a Kalashnikov when he came to Brussels in early April, but claimed he never delivered it.
'Set of scattered deductions'
Upon his arrest, Nemmouche was found with a nylon jacket with gunshot residue, as well as a computer in which investigators found six videos claiming the attack with an off-camera voiceover thought to be Nemmouche.
The presiding judge singled out the evidence on his jacket which she said "contained exclusively traces of DNA from Mehdi Nemmouche".
In total, the prosecution said it had identified 23 pieces of evidence pointing to Mehdi Nemmouche, who also physically resembles the shooter seen on the museum's surveillance video.
The verdict said: "The defence limited itself to outlining a set of scattered deductions without ever elaborating on them." It added that Bendrer, by supplying the weapons, was aware of aiding a crime committed by "a longstanding radical," alluding to Nemmouche.
Prosecutors say the attack was the first carried out in Europe by a jihadist returning from fighting in Syria, 18 months before the November 13th Paris attacks of 2015 in which 130 people were killed.