The fire started at about 2 a.m. in a 16-unit apartment building at 5225 Samson Boulevard, in the Chomedey district.
Laval police Const. Franco Di Genova said the building was evacuated as a precaution and that's when officials discovered a small marijuana grow-op.
He said about 200 one-foot tall plants were found in a second storey apartment.
Di Genova said witnesses described seeing two people run away shortly before two or three small fires started on two different storeys of the building.
Firefighters reported finding traces of accelerant.
None of the building's residents were injured.
Investigators are on the scene to determine what was used to start the fires.