Police said the incident happened at around 8.10pm on October 23 when a man entered the McColls shop in Tunley Holme through the front armed with a baseball bat. He made threats towards a member of staff and demanding cash from the till.
He grabbed some of the cash before a second member of staff was able to activate a panic alarm.
The robber made off through the front door of the shop, concealing the baseball bat under his clothes as he ran.
He was wearing a hat and face covering, black coat and tracksuit bottoms.
Det Con Rachael Taylor-Gore said: “The staff in the shop have been put through a very frightening ordeal which has left them extremely shaken.
“We are now determined to catch the person responsible and would appeal to anyone with information about it to contact us.”
Call police on 101 quoting log 1418 of October 23.