Frenchman Mehdi Nemmouche was convicted by a jury on Thursday in Brussels for the murder of four people at the city's Jewish Museum on May 24, 2014.
The 33-year-old was found guilty of committing "four terrorist murders" when he killed his victims in cold blood in less than 90 seconds. An Israeli couple and two museum workers were killed in the attack.
Sentencing will be announced at a later date, but he could face up to 30 years in prison.
The jury also found Nemmouche's accomplice, Nacer Bendrer, guilty of supplying the revolver and assault rifle used in the attack.
Jewish group slams 'reprehensible tactics' of defense team
The 33-year-old defendant denied the charges and told the court on Tuesday that he was "tricked."
His lawyers argued he had been caught up in a plot organized by the Israeli intelligence agency, Mossad. Nemmouche's legal team claimed that video footage of the attack was faked and that the defendant was framed for the killing of two Mossad agents.
The defense's arguments outraged the families of the victims as well as survivors of the attack.
The European Jewish Congress chief, Moshe Kantor, condemned "the use of reprehensible tactics and conspiracy theories [of] the defense lawyers."
Sparking fears of attacks in Europe
Thursday's verdict made Nemmouche the first European foreign fighter to be convicted of terrorism offenses.
Prosecutors argued that Nemmouche fought with the militant "Islamic State" (IS) group in Syria before returning to Europe and carrying out the attack.
The shooting at the museum has been regarded as the first to make clear the threat posed by radicalized militants returning to their home countries in Europe after fighting in Syria's war.
European countries have been struggling over what to do with foreign fighters, particularly on how to take them back and whether or not to strip them of citizenship.
The European Union opposes the death penalty, but IS fighters in jail in Iraq or Syria could be tortured or executed if they remain there.
Since many EU countries don't have extradition treaties with either Syria or Iraq, it's proving difficult to prove the identities of foreign fighters and to gather evidence that could be used to try them in European courts. There's also the separate issue of what to do with the wives and children of European jihadis.
rs/sms (AFP, AP, Reuters)