It happened around 1:15 p.m. on March 2 at the Starbucks on the 1200 block of Market Street.
Police say the gunman approached a 19-year-old female employee.
They say he demanded she fill a bag with money while threatening to shoot her.
When the employee backed away from the suspect, police say he fled the store without any money.
He was last seen west on Market Street towards 13th Street.
The suspect is described as a black male, late 30s, 6', thin build, scruffy beard, wearing a black knit hat, dark colored hooded sweatshirt, and dark colored pants.
Anyone with information should contact Central Detective Division at 215-686-3093/3094.