It happened at the Citizens Bank branch at 2702 Kirkbride Street around 11:30 a.m. Sunday.
Police say a woman went into the bank and gave the teller a threatening demand note.
She obtained an undisclosed amount of cash, then fled on foot eastbound on Kirkbride Street.
The suspect is described as a white female in her 30's, 5'5" tall, 130 pounds, wearing a military-style camouflage cap with blond hair pulled up inside the cap, sunglasses, a bluish green t-shirt, and khaki shorts.
The FBI says the suspect is considered armed and dangerous.
Anyone with information is asked to call the FBI at 215-418-4000 or the Philadelphia Police Department.
There may be a reward for information leading to this subject's capture, and tipsters may remain anonymous.
