The started fire after midnight at a home near the corner of Tommy Douglas and Abraham DeSola streets, near the Hippodrome.
Montreal police Const. Daniel Lacoursière said the fire spread to neighbouring buildings, forcing about a dozen people out of their homes.
"Since the Montreal fire department wasn't able to determine the origin of the fire, as protocol warrants, the investigation was handed over to the Montreal arson squad," Lacoursière said.
Radio-Canada is reporting that witnesses saw someone throw an incendiary device at the building before fleeing. Police would not confirm the information.
No one was injured.