It happened around 10:26 a.m. on Monday at the store at 121 E. City Avenue in Bala Cynwyd.
Investigators say the man approached the teller counter at the Citizens Bank inside the store, demanding $50 and $100 bills.
The teller gave him the money, and he then walked out of the store.
The suspect is described as a black male in his 40's standing about six feet tall.
He was wearing a blue or black pullover sweater, sunglasses and a navy blue knit cap.
Anyone who recognizes the man is asked to call Lower Merion Police at 610-649-1000.