The invention relates to a knee-joint orthosis which comprises a fastening means (1),and at least one stable guide element (6) that is arranged to bear laterally on the patella by way of a lateral base part (7) from which medially directed attachments (8) extend proximally and distally of the patella and are secured to strap sections (10, 12) on the orthosis in the area of the thigh and of the lower leg in such a way that a medial tensile force is exerted on the attachments (8). By virtue of the fact that the strap sections (10, 12) are designed with an elasticity, that an elastic strap (19) is connected to the base part, and that the elasticity of which strap (19) is greater than the elasticity of the strap sections (10, 12), the elasticities of the strap sections and of the strap (19) are chosen such that, during a flexion of the joint (14) from the extended position, the laterally directed strap (19) first stretches at least quite predominantly, and a stretch limiter (23), providedfor the laterally directed strap (19), is active at a predefined flexion limit angle of the knee joint, and such that, for a greater flexion of the knee joint (14), the medially directed strap sections stretch at least predominantly.