They say is happened around 17.50, and was an attempted robbery.
The man is described as wearing a grey tracksuit, white trainers, with a black hat. The Police say he demanded money from the female worker and brandished a knife before leaving the shop empty-handed. No one was hurt.
Anyone with information is asked to contact the Police on 612612 or Crimestoppers on 0800 555 111, and islanders are being urged not to approach the man, if they see him, but to call the Police instead.