It happened around 10:45 Saturday at the branch inside ACME supermarket on the 100 block of East City Line Avenue.
Police say the male suspect walked inside and handed a note to the teller demanding money.
After getting an undisclosed amount of cash, he fled in a dark colored SUV.
The suspect is described as a black male in his late 20s, approximately 6'2" with a goatee or possibly a beard.
He was last seen wearing a navy winter coat with fur trim on the hood, blue jeans, a grayish baseball cap with writing, dark gloves and dark shoes.
Anyone with information is urged to contact the FBI at 215-418-4000 or Lower Merion Police at 610-649-1000.