The woman was found unconscious in an apartment on the second floor of the building located on Duval Avenue .
"Unfortunately, we found one victim inside the housing [hidden] under rubble," said Fire Chief Christian Paradis .
Her death was confirmed later at the hospital.
At least a dozen tenants trapped inside the building were rescued from the quickly-spreading flames by firefighters. The fire started shortly after 6 a.m. ET in one of the three-storey building's 16 units.
More than 50 firefighters responded to the blaze.
Martin Côté, a resident of the building, said the fire was ignited by a small electric heater on a mattress.
He said he tried to put out the flames, emptying an entire extinguisher, but the fire had already spread.
"We weren't able to see anything," he said. "There was was black, black smoke. I was really shaken."
Investigators have not released the official cause of the fire.
The Red Cross was called in to help displaced residents.