The masked man demanded cash as he threatened a 56-year-old woman working at the Ladrokes Bookmakers in Dubbs Road, Port Glasgow with a weapon.
He then made off with the money, leaving the woman traumatised but unhurt, Police Scotland said.
Detectives are investigating the incident, which took place at about 9.15pm on Saturday. They described the man involved as being white, approximately 6ft tall, and of slim to medium build.
He was wearing a black hooded jacket, blue hooded jumper, grey tracksuit bottoms and white trainers at the time. His faced was covered with a black balaclava.
Detective Inspector George McAllister, of Greenock CID, appealed for any witnesses to come forward, saying: "I am anxious to speak to anyone who was in the area of Dubbs Road after 9pm on Saturday night.
"They may have seen the suspect hanging about in the area prior to the robbery. It is believed he made off on foot and I would urge anyone who saw this man to contact police."