Firefighters were called to the building at about 6 p.m. yesterday.
The woman was found with burns on most of her body and rushed to the hospital.
Longueuil police Const. Patrick Barrière said hospital officials confirmed the woman died at 10:30 p.m. last night.
Longueuil police are investigating the fire, but Barrière said it was likely an accident.
He said police suspect the woman started the fire when she dropped a lit cigarette.