Abstract: A mono-condylar knee joint prosthesis that includes a femur part having a convex running surface. The knee joint prosthesis further includes a tibia part having a sliding surface which faces the femur part and an intermediate part which is introduced between the femur part and the tibia part during implantation. The intermediate part has two bearing surfaces, namely a first, concave bearing surface for cooperating with the convex running surface of the femur part as well as a second bearing surface for cooperating with the sliding surface of the tibia part. A boundary surface is provided at the tibia part which bounds the sliding surface of the tibia part and projects pointing in the direction towards the femur part in order to be able to cooperate with a side surface of the intermediate part.