Police say he walked into a Citizens Bank branch on the unit block of East Baltimore Pike in Media at 4:15 p.m.
He demanded money from a teller who handed over some cash. The suspect then ran off and remains on the loose.
The suspect is described as a black male, 30-40 years of age, 6'0" to 6'2" tall, heavy build, with short or close-cropped hair, a full beard, wearing dark sunglasses, a long white t-shirt, and tan or grey pants.
Anyone with information is asked to call the FBI at 215-418-4000, the Delaware County Criminal Investigation Division at 610-891-4700, or the Media Police Department at 610-565-6656.
The suspect is considered armed and dangerous.
