Police have appealed for witnesses following the incident at the William Hill shop in Beith, North Ayrshire.
The man walked into the shop on Eglinton Street at around 9pm on Wednesday.
He fled with a three figure sum of money after threatening the female assistant with a weapon.
The suspect was wearing a motorcycle helmet a black motorcycle jacket and jeans.
Detective Sergeant Graham Duncan, of Saltcoats CID, said: "We believe that the suspect left on foot however he was wearing a motorcycle helmet and motorcycle jacket and may have had access to a motorbike.
"I would ask anyone who was in the area around the relevant time and saw someone answering the description of the suspect to come forward."
Anyone with information is asked to telephone Saltcoats CID on the non emergency number 101 or Crimestoppers on 0800 555 111.