Purpose There are controversies about combining distal tibia medial opening-wedge osteotomy (DTMO) with fibular valgization osteotomy (FVO) when performing supramalleolar osteotomy (SMO) for medial ankle osteoarthritis. This study aimed to assess the effect of FVO on the coronal translation of the mechanical axis by comparing the improvement of radiological indices after DTMO with and without FVO. Methods Forty-three ankles (mean follow-up: 42.0 months) were reviewed after SMO. Among them, 35/43 (81.4%) underwent DTMO with FVO, while 8/43 (18.6%) underwent DTMO only. To evaluate the effect of FVO radiologically, the medial gutter space (MGS) and talus centre migration (TCM) were measured. Results Post-operatively, MGS and TCM were not significantly different after DTMO only and DTMO with FVO. However, the improvement of MGS was significantly higher in the combined FVO group (0.8 mm (standard deviation SD 0.8 mm) vs. 1.5 mm (SD 0.8 mm); p  = 0.015). More lateral translation of the talus was achieved in the FVO group (5.1 mm (SD 2.3 mm) vs. 7.5 mm (SD 3.0 mm); p  = 0.033). However, the changes in the MGS and TCM were not significantly correlated with the clinical outcomes ( p  > 0.05). Conclusion Our radiological evaluation confirmed a significant medial gutter space widening and lateral talar translation after the addition of FVO. The SMO with fibular osteotomy allows greater shifting of the talus and, therefore, the weight-bearing axis.