A new planar robotic exoskeleton for upper-limb motor assessment has been developed. It provides independent control of a user’s shoulder, elbow, and wrist joints in the horizontal plane. The lightweight backdriveable robot is based on a novel cable-driven curved track and carriage system that enables the entire mechanism to be located underneath the user’s arm. It has been designed to extend the assessment capabilities of an existing planar robotic exoskeleton. This paper presents the design and performance of the new robot.