Supramalleolar osteotomy (SMO) has been suggested as an effective salvage treatment for varus ankle osteoarthritis. To identify the prognostic factors affecting the correction angle changes after SMO, a total of 53 consecutive patients (58 ankles) were evaluated retrospectively. Clinical and radiologic outcomes were evaluated, and statistical analyses were performed to identify the prognostic factors associated with the clinical and radiologic outcomes. The mean visual analogue scale scores and the American Orthopaedic Foot and Ankle Society scores improved significantly at the final follow-up (both p <.001). The mean tibial-ankle surface (TAS), talar tilt (TT), and tibial-lateral surface angles improved significantly after surgery, compared with the preoperative assessments (all, p <.001). However, at the final follow-up, these angles had changed significantly, compared with their immediate postoperative values (all p <.001), and the changes in the TAS and TT angles significantly influenced the clinical outcomes at the final follow-up (both p <.05). Male sex, high body mass index (≥26.4 kg/m2), and the existence of the lateral cortex breakages were significantly associated with the changes in the TAS and TT angle (all p <.05). Therefore, surgeons should consider these prognostic factors before performing SMO.