It happened just before 6:30 p.m. in the 300 block of Market Street.
Police say the suspect presented the tellers with a demand note and fled on foot after receiving an undetermined amount of cash.
He is described as a heavyset black man, about 6 feet tall, with a beard. He was wearing a brown hoodie and black cap.
There were no injuries.
Anyone with information is asked to contact Philadelphia Police.