Sûreté du Québec spokesperson Ronald McInnis said police were called to a group home for people with mental illness on 44nd Street just after 11 p.m., after caregivers at the home found one of the residents had gone missing.
By the time police arrived, the missing man had returned, wielding a knife.
"He was angry, and at this moment, he took the knife and [went for] the police officer," McInnis said.
McInnis said it's at that moment that, feeling threatened, the officer shot the man in the lower body.
The man was taken to hospital, although he is expected to make a full recovery.
The officer was treated for shock.
By law, whenever someone is hurt or killed during a police operation in Quebec, the investigation is turned over to another police force.