An official said the suspect has been handed to anti-terrorist investigators and could be held at least until tomorrow under French counter-terrorism law. The man was arrested on Friday during a customs inspection at a railway and bus station in Marseille.
The man was found to have a revolver and an automatic weapon of the same type used in the Brussels shootings on May 24. The official said ballistics analyses are under way to determine if it is the same weapon.
The man had arrived in Marseille on a bus from Amsterdam that had stopped in Brussels, she said.
The Paris prosecutor was expected to give a news conference today.
The killings, which came on the eve of European Parliament elections in which far-right parties had a strong showing, led Belgian officials to raise anti-terror measures.
Video of the attack showed an athletic man with a cap walking determinedly into the small Jewish Museum. The whole assault took a minute at most.