It happened at the Associated Bank on North Main and Halstead.
Police say the suspect, a white male wearing all dark clothing and a ski mask, entered the bank with a handgun about 1:00 p.m. Thursday afternoon.
He took some money from the teller.
Seven employees, but no customers, were in the bank at the time of the robbery, and no one was injured.
The robber left on foot. Police think he hopped over a fence and escaped in a vehicle nearby. Investigators are trying to figure out what type he used to make his getaway.