Osteoarthritis is usually found in weight-bearing joints. Since medial knee joints bear most of the loading during gait, osteoarthritis is especially common in these joints. For early osteoarthritis patients, unloading or increasing muscle strength through exercise is the most fundamental healing approach. The use of assistive devices during exercise can relieve pain and thus increase patient motivation or improve on the duration. However, long-term use of assistive devices may cause discomfort or even side effects and in turn make matters worse. Hence, the purpose of this study includes (1) Developing an innovative assistive device that reduces medial knee loading; (2) Aiming for this device to relieve knee pain caused by OA; (3) Aiming for this device to be free of knee and peripheral muscle movement restrictions and thus increase comfort. To design an innovative assistive device, this research obtains design criterions through reviewing existing products and expert focus groups. Design concepts are generated in co-design activities following these criterions and eventually converged into one final design with morphological charts. Finally, prototypes are built according to the final design. The results of the experiments show that this innovative assistive device can significantly reduce medial knee loading on both legs, and that the satisfaction scores of design criterions are able to reach an acceptable level.