Authorities say it happened just after 7:00 p.m. in the 8600 block of Germantown Avenue during "Stage and Doe Night", an annual promotion where stores stay open late for holiday shopping.
The man pointed a gun at the teller and demanded cash before fleeing on foot with an undetermined amount of cash.
Police say he was last seen running east on Evergreen from Germantown Avenue. He is described as a tall black man in his 40's carrying a duffle bag.
Authorities are hoping surveillance video will help them with the investigation.
They are asking anyone with information to contact police.