It happened around 2:30 p.m. Tuesday at the PNC Bank on the 3600 block of West Chester Pike in Newtown Square.
Police released surveillance images showing the robber, wearing a surgical mask, walking up and threatening a teller with a gun.
The robber got away with an undisclosed amount of money.
Police did not report any injuries.
The suspect is considered armed and dangerous.
Anyone with information should call the Newtown Township Police Department at 610-356-0602 or contact the FBI at 215-418-4000 or tips.fbi.gov.