It happened around 3:00 Friday afternoon at 31 South Chester Road.
Police say the male suspect walked into the bank and approached the teller with a hand-written note implying that he was armed and demanding money.
The suspect told the teller that he had a gun and reportedly said, "Don't do anything stupid."
The suspect got away with an undisclosed amount of money and fled on foot towards Rutgers and Cornell avenues.
No one was injured.
Police say the alleged bandit is described as an African-American male, in his mid to late 20s, with a thin build, and approximately 5'6" to 5'8" tall.
He was last seen wearing a dark coat, winter style hat and he had a large amount of facial hair.
Anyone with information is asked to contact the Swarthmore Borough Police Department at 610 543-0123.