Police say staff at the Grosvenor Road premises were threatened during the attack.
Staff were preparing to close up for the evening when the robbery happened around 8.50 pm last night.
The man fled with a sum of cash.
No one was injured during the hammer-wielding incident.
Police say the armed robber is described as wearing a grey hoodie and tracksuit bottoms.
He also had a scarf over his face to hide his identity.
Anyone with information about the robber is asked contact detectives on the 101 non emergency number.