It happened at about 20:30 GMT on Saturday at the Antrim Road shop.
Two men wearing hooded tops made a purchase, then one asked the assistant for another item. When she opened the till, one of the men brought out a knife.
He lunged towards the woman, jabbing the knife in her direction. The second man them removed cash from the till.
Both were challenged by a male member of staff as they left and one of the robbers took the knife out again. They then fled on foot.
Neither member of staff was injured, but police said they were left very badly shaken.
Detectives have appealed for witnesses to contact them.
A woman working in an off-licence has been threatened with a knife during a robbery in north Belfast.