Quebec provincial police officers noticed smoke coming from the 12-unit building around 4:15 a.m. on Sunday and found the body upon entering the premises.
Police spokesman Ronald McInnis said an autopsy will be conducted in Montreal.
Police and firefighters evacuated 15 people from the building. They were taken in by the local Red Cross and put up at the Hôtel Rimouski.
Rimouski Fire Chief Bertin Santerre said the cause of the fire is under investigation.