[Purpose] This study investigated the effects of proprietary foot orthotics in young adults with flatfoot to determine changes in the ankle joint angle in the coronal plane during the midstance phase. [Subjects and Methods] The subjects were 15 college students diagnosed with flatfoot. Changes in the ankle joint angle in the coronal plane in the midstance phase were measured using the Vicon Motion System before and after use of the orthotic. The data were analyzed using Statistical Package for the Social Sciences Win 16.0. [Results] The subjects showed significant increases in left and right ankle joint angles in the coronal plane during the midstance phase of the gait cycle after use of the orthotics. However, the difference between the left and right ankle joint angles showed no significant change, even though the difference increased after use of the orthotics. [Conclusion] Young adults with flatfoot showed increased ankle joint angles after use of the orthotics. This suggests that orthotic footwear can shape the plantar arch and affect the ankle joint, and that constant use of orthotics would cause a dynamic change in normal walking.