It happened in the Railway Street, Murlough Upper area of Newcastle.
Around 12 mid-day a male entered the shop and took a knife from a display and threatened the female shop assistant.
He demanded that she hand over the money from the till. He made off with a sum of money.
The shop assistant was not injured however she was left very badly shaken.
A 24 year old man has been arrested in relation to this incident. He is currently in custody.
Detectives in Downpatrick would ask anyone who may have any information to contact them at Downpatrick on the new non-emergency number 101.
Or, if someone would prefer to provide information without giving their details, they can contact the independent charity Crimestoppers and speak to them anonymously on 0800 555 111.