Investigators say the man walked into the Citizens Bank branch in the 6900 block of Torresdale Avenue on Friday after 1:30 p.m.
He handed one of the tellers a threatening demand note and fled with an undisclosed amount of cash.
Police say they are looking for a black man in his mid-20s, about 5'9" tall with a slim build, medium complexion and a beard.
He was wearing a blue hooded sweatshirt, black pants, and black gloves.
Anyone who sees the suspect is asked not to approach him and to call 911 immediately.
Anyone with information on the crime is urged to contact Philadelphia Police at 215-686-TIPS (8477), text a tip to PPD TIP (773847) or through PhillyPolice.com.