Montreal police said emergency crews were called to the scene after an alarm went off in the Jarry Street building, located near Pascal-Gagnon Street, just after midnight.
Const. Anie Lemieux said when police arrived at the scene, they found one of the windows of the ground-floor business broken and a fire burning inside.
Lemieux said investigators are looking into what caused the fire.
"Was it something that was thrown? Is it something that was poured? This is what investigators will be looking into," she said.
Firefighters quickly put out the fire.
Investigators were at the scene later to investigate further.
Damage to the gym was significant, and some adjacent businesses sustained smoke damage.
Antoinette Volco, a hairdresser at Coiffure Omorfo, said the salon adjacent to the gym had to be closed for the day. She said the gift basket shop next door was also damaged.
"She has more damage, all of her baskets are black, her counters are black. The baby stuff -- all the baby teddy bears – they all smell really bad," she said.
Police say they do not have any suspects.
In October, a gym in Villeray was also the target of a criminal fire after someone smashed a window and tossed in a Molotov cocktail. The damage was minor in that incident.
Police have made arrests in connection with that fire, but said the investigation is still ongoing.They say they've found nothing to indicate the two incidents may be linked.