It happened at 4:00 p.m. Friday at the PNC Bank branch in the 2400 block of Welsh Road.
Authorities say the man walked into the bank and presented a threatening demand note to a teller.
After obtaining an undisclosed amount of cash, he fled on foot. No one was injured.
Investigators are looking for a black male in his 30s, 5'10" tall, medium build with thin facial hair. He was wearing a black and white NY Nets baseball cap and a plain black t-shirt.
Anyone with information Philadelphia Police or the FBI at 215-418-4000.
The FBI says there may be a reward for information leading to the bandit's capture. Tipsters can remain anonymous.