The robbery happened just before 11:00 a.m. at a TD Canada Trust bank branch on Roland-Therrien Boulevard.
Police say the suspect sprayed people in the bank with pepper spray. Some had to be taken to hospital for treatment.
The suspect left with an undisclosed amount of money.
The suspect is between 25 and 30 years old and wearing a dark winter coat.
Investigators don't know if the man acted alone or with an accomplice.