We aimed to develop a digitally portable hand assessment instrument, named the C-shaped ring, and to assess the severity of the impairment of hand function using C-shaped-ring-based data for patients with stroke. The C-shaped ring was designed as a C-shaped structure. An inertial measurement unit and an infrared sensor were installed to collect the movement and force torque data. Clinical examinations including hand strength (pinch strength and grip strength), Purdue Pegboard Test (PPT), and Box and Block Test (BBT) were assessed for the outcome measure. Predictors were data collected during the grasping and release of the C-shaped ring with the affected hand. A total of 30 patients participated in this study. The multiple regression analysis explained 41.9, 66.7, 76.4, and 61.2% of the variability in pinch strength, grip strength, and the PPT and BBT results, respectively. In this research, a substantial amount of the variance in the fine dexterity function of C-shaped-ring-based data was observed. The C-shaped design of the C-shaped ring assisted the patients with stroke in performing the grasp-and-release task with the proper hand position, and the spring mechanism allowed the patients to exert grip force actively and release the grip passively.