It happened at 9:35 a.m. at the Republic Bank branch in the 1800 block of Market Street.
Police say the suspect walked into the bank, handed the teller a demand note, then fled east on Market Street with an unknown amount of cash.
His face was recorded by a bank surveillance camera.
Investigators are looking for a dark-complected black man about 30 years old, 6 feet tall, with a slender build and a closely shaved goatee beard, wearing a black knit hat, brown waist-length jacket and sunglasses.
Anyone with information about the crime or the suspect is urged to call the FBI/Philadelphia Police Department Violent Crimes Task Force at 215-418-4000.