One of the main challenges for the elderly is insufficient lower limb strength during sit-to-stand movement, which may be improved by supporting the joint externally. Existing lower extremity exoskeletons use perfect revolute joints as knee joints, which do not match with human joint biomechanics. They also require a complex control system to produce the required torque at the corresponding joint angle. In this study, a knee support device using four-bar joint mechanism and hydraulic artificial muscle (HAM) was designed. A previously proposed four-bar linkage joint was modified to accommodate the HAM. In addition, the Angled Bar was proposed to exploit HAM’s force-contraction relationship to generate the desired torque at the corresponding angle only by applying constant hydraulic pressure without the use of a complex control system. The device was able to generate a maximum output of 126.55 Nm torque at 100∘ knee joint angle during loading and 70.69 Nm torque at 100∘ during unloading at 3 MPa pressure. The root-mean-square error of the knee extension torque curve was 13.01 Nm. Experiment with a healthy participant showed significant reduction in muscle activity with the assist from the device. The maximum processed EMG signal with and without assist were 52.10 and 20.93 μV, respectively.