Two armed men stormed into the fast food restaurant in the Völkermarkter road at 1:45 am, according to the National Police Directorate of Carinthia.
The employees and six customers were held at gunpoint, and were forced to lie on the floor.
The two perpetrators fled on foot after stealing the restaurant's cash.
Police immediately started a manhunt for the two armed robbers, who are described respectively as being between 1.9 and 2 metres tall, and 1.6 and 1.65 metres tall.
The larger man was wearing a bright hooded jacket, grey or light blue trousers, and was wearing a dark backpack. The smaller man was wearing a dark jacket, with a white or yellow stripe.
A few weeks ago another McDonald's restaurant in Klagenfurt was also been robbed, although a link between the two crimes is not yet confirmed.