Christine Coulombe, a spokeswoman for Quebec provincial police, says the 26-year-old man was interrogated by detectives Saturday night.
Coulombe says he could face charges including arson with disregard for human life.
The police officer would not comment on the details of the case but says clues found at the scene led investigators to believe the fire was deliberately set.
The man was stopped by Granby police two hours after the fire, which required 100 patients to leave their rooms.
No one was injured in the incident.
Denis Dube, a hospital spokesman, said six of the patients — including a pregnant woman — were transferred to other hospitals in the area because they needed special care.
The others were relocated on different floors of the hospital once it was determined it was safe.
Dube said there was a strong smell of smoke when the evacuation was ordered.
He said the situation had returned to normal Saturday evening and patients had been readmitted to their original rooms.