Stretching is a widely used method for managing spasticity. It involves elongating muscles and soft tissues to improve flexibility and range of motion. We aimed to evaluate the effectiveness and usability of static stretching utilizing a Hand–Wrist Stretching Device (HWSD) that we developed for managing spasticity and improving the motor function in individuals with chronic post-stroke hemiparesis. A repeated experiment was conducted to assess the effectiveness and usability of the HWSD. The HWSD includes a forearm holder, a frame, a finger holder, adjustable hinges, straps, and thumb stretchers. The Modified Ashworth Scale (MAS), the Box and Block Test (BBT), and a subjective usability questionnaire were used for evaluation. The results of applying the MAS revealed a significant difference with time and stretching angle for the elbow (p < 0.05). Both the elbow and wrist showed a significant decrease in MAS scores, and the BBT results demonstrated a significant improvement after stretching (p < 0.05). The subjective perception ratings consistently remained above 4.0. The HWSD effectively relieves hand spasticity and improves hand mobility in chronic stroke patients. Subjective perception ratings were positive. In future studies, the application of HWSD to patients with various forms of spasticity can be explored and its long-term effects investigated.