This article introduces the design and the experimental validation of the Trackhold, a novel mechanical motion-tracker for upper limb physical rehabilitation. The Trackhold is based on a passively balanced mechanism that can approximately relieve the weight of the patient’s arm regardless of the position. The system features a novel kinematic architecture with large workspace and custom developed joint sensors providing accurate real-time measure of the upper limb posture. The design approach of the device, which went through kinetostatic and dynamic analyses, is presented and details on the employed mechatronic solutions are provided. A prototype of the Trackhold has been fabricated and functionally validated.