It happened at an off licence in the Ballymacash Road area of Lisburn yesterday, Monday, January 12.
Detective Inspector Chris Millar said: “A man armed with a knife entered the premises shortly after 11am on Monday morning and threatened a member of staff.
“A sum of money was handed over and the man then left the scene on foot in the direction of the Rathvarna Estate.
“He was described as being around 6’ tall, and was wearing a black hat, a green jacket and had a scarf covering his face. The staff member was not injured.
“I would appeal to anyone who noticed any suspicious activity in the area or who has any information to contact CID in Lisburn on 101.
“Information can also be given anonymously through the charity Crimestoppers on 0800 555 111.”