In this article, a novel lower extremity gait rehabilitation device with a single actuator is presented. The proposed device, Mech-Walker, utilizes the single-degree of freedom eight-bar Jansen's mechanism. The mechanism is synthesized to generate the ankle trajectory of human gait, relative to the hip, in terms of both position and time. Two mechanisms, one for each leg, are applied reciprocally and are mechanically synchronized to guarantee symmetric gait. A custom designed seat-type weight support system mounted over a commercially available treadmill is also introduced. It not only supports the user and the mechanisms but also links the mechanism's base to the subject's hip. This vertically movable base allows ground contact proprioception for the user which is crucial in gait training. As this is a nonanthropomorphic design interacting with the leg only at hip and ankle via seat and footrest, respectively, it does not need to be aligned with the subject's joints, which facilitates the donning and doffing of the device. Adjustability of mechanism to different users has also been considered to provide an effective training tool. A prototype is manufactured, and pilot study with a healthy subject is conducted. Due to ease of control, cost-effectiveness, and high intrinsic safety, the proposed system potentially offers a novel tool for gait training.