Police say the robbers, armed with guns, entered a flat on the Ballyloughlin Road in Castlewellan in the early hours of Monday morning.
One of two occupants in the flat at the time was struck on the face with a gun during the incident.
The victim’s injuries are not believed to be life-threatening.
The masked intruders later fled a sum of money.
Detectives at Downpatrick CID have appealed for anyone with information about the robbery and assault to contact them on 0845 600 8000.