The fire spread quickly through the Residence du Havre in L'Isle-Verte, a small town about 140 miles northeast of Quebec City, the QMI agency reported. It began around 12:30 a.m.
Many residents remained missing late Thursday. Authorities said 14 were hospitalized, including one who was transported to Quebec City and described as being in "grave condition."
"A woman on the second floor was screaming, she went out on the balcony," Mario Michaud, who lives across the street, told the agency. "Her son went to get a ladder, but he couldn't go to get her. She burned to death."
After the fire was extinguished at around 6 a.m., firefighters worked through the day, searching the remains of the building for the dead.
About 60 people lived in the building, many of them unable to get around without a wheelchair. During a recent inspection, the owners said 37 of the residents were older than 85.
"We even had a patient who had thrown himself from an upper floor to save himself from the flames," paramedic Harold Lemelin told QMI.