The bus had been bound for Paris and was carrying 30 passengers and two drivers when it crashed into the support pillar of a highway overpass, a police spokeswoman said.
"An investigation is under way, but at the moment the causes of the crash are unknown," she said, adding that the bus had come from the southern Moroccan city of Tiznit.
Thirteen other passengers were slightly injured, and four had already left hospital. One of the two drivers was among those killed and the other was seriously injured.
Hundreds of thousands of Moroccans work in France and many regularly return home to see their families.