BACKGROUND: Double-level osteotomy is hypothesized to provide superior biomechanical outcomes compared to single-level osteotomy for medial knee osteoarthritis with double-level deformity. However, its advantages remain underexplored. METHODS: This retrospective study analyzed 20 knees treated with medial open wedge high tibial osteotomy alone or combined with lateral closed wedge distal femoral osteotomy. Patients were categorized into three groups: Single-level osteotomy for single deformity, single-level osteotomy for double deformity, and double-level osteotomy for double deformity. Radiographic parameters, including Hip-Knee-Ankle angle, mechanical lateral distal femoral angle, medial proximal tibial angle and joint line convergence angle, and gait biomechanics using nine-axis inertial measurement units were evaluated pre- and postoperatively. FINDINGS: The postoperative mechanical lateral distal femoral angle was better corrected in the double-level osteotomy for double deformity group compared to the single-level osteotomy for double deformity group (P = 0.012). Peak thigh varus acceleration was significantly reduced in both the double-level osteotomy for double deformity and single-level osteotomy for single deformity groups (P = 0.014), but not in the single-level osteotomy for double deformity group. No significant differences were observed in medial proximal tibial angle among groups. INTERPRETATION: Double-level osteotomy demonstrated superior biomechanical and radiographic outcomes in medial knee osteoarthritis with double-level deformity, emphasizing its role in mitigating joint line obliquity and in optimizing dynamic knee stability.