A knee orthosis (1) for support of a knee joint in the case of instability comprises a thigh portion (4), which is attachable on the thigh, and a lower leg portion (5), which is attachable on the lower leg, which lower leg portion (5) is provided with splint elements (12), which are connected in an articulated way, via articulation means (13), to hinge plates (14) attached on the thigh portion (4). The lower leg portion (5) has a lower half shell (9), which in the region remote from the knee is borne in the splint elements (12) in a way pivotable about a pivot axis (15). Via a longitudinally stable, flexible traction (17) the lower half shell (9) is pushed ventrally against the lower leg, the pressure increasing with bending of the knee. An optimal support of the knee joint is thereby achieved, especially when the cruciate ligaments are injured.
