Two men armed with a blunt weapon targeted the Scotmid store in Greenock, Inverclyde, on Monday.
Police said the staff, an 18-year-old man and a woman aged 26, had been left traumatised by the robbery.
The men – wearing "ski-type" masks – entered the shop on Murdieston Street and demanded money from the assistants before assaulting them. The were not seriously hurt.
Detective Inspector Julie Henry, of Strathclyde Police, said: "This was a horrendous attack on two innocent people who were going about their jobs when they were subjected to a violent assault.
"We believe the men involved in the attack had access to a small, light-coloured vehicle which they made use of when leaving the scene of the robbery.
"I would like to speak to anyone who was in the area of the Scotmid store at the time of the incident, or anyone who was in the vicinity of Murdieston Dam around 10pm."
The men are described as between 5ft 2in and 5ft 6in and of heavy build.