Abstract: An orthopaedic knee brace comprising an upper arm and a lower arm securable to a wearer's leg above and below the knee joint. There is a pivotable joint at the ends of the arms to allow flexing of the knee by pivoting movement of the arms. A locking mechanism is provided to control pivoting of the joint to lock one of the upper or lower arms relative to the joint while automatically unlocking the other arm to permit pivoting movement of the other arm about the joint. The joint and locking mechanism co-operate to form a light weight structure that operates smoothly to imitate the complex bending motion of the knee while supporting the knee.