In a knee-joint orthosis for guiding the patella of a patient during the transition from an extended position to a flexed position of the knee joint and vice versa, with a fastening means (I) for fastening to a thigh and a lower leg, with at least one laterally arranged stabilizing element that takes up a flexion of the knee joint and bears on the thigh and lower leg, and with a dimensionally stable guide element (6) that can be held in position by an arrangement of tensioning straps and 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).