INTRODUCTION: The impact of medial unicompartmental knee arthroplasty (MUKA) on ankle alignment is not well-studied. This study aims to investigate the changes in ankle alignment following MUKA and identify the influencing factors. MATERIALS AND METHODS: A retrospective analysis included 175 patients undergoing MUKA between 2018 and 2020. Patients were categorized into varus (n = 113) or valgus (n = 62) ankle groups based on preoperative ankle deformities. Preoperative and postoperative full-length standing radiographs were used for radiographic measurements. RESULTS: Following MUKA, significant differences in the change in tibial plafond-talus angle (PTA) were observed between the groups, with the varus ankle group showing a change of -0.71 ± 0.82° and the valgus ankle group showing a change of 0.08 ± 0.94° (p <
0.001). In the varus ankle group, the tibial plafond-ground angle (PGA) increased from -3.65 ± 4.22° preoperatively to -0.51 ± 4.52° postoperatively (p <
0.001), talus-ground angle (TGA) increased from -5.28 ± 4.32° to -1.32 ± 4.74° (p <
0.001), and PTA decreased from 1.52 ± 1.04° to 0.81 ± 1.12° (p <
0.001). In the valgus ankle group, PGA increased from -5.44 ± 4.39° to -1.43 ± 4.63° (p <
0.001) and TGA increased from -4.55 ± 4.24° to -0.59 ± 4.47° (p <
0.001), but PTA did not show a significant change. Ankle alignment change significantly correlated with preoperative joint line convergence angle (JLCA), preoperative medial proximal tibial angle (MPTA), preoperative PGA, preoperative TGA, preoperative PTA, hip-knee-ankle angle (HKA) changes, and bearing thickness. CONCLUSIONS: MUKA significantly corrects the majority of ankle alignment towards a more neutral position. The extent of ankle alignment correction is influenced by preoperative knee and ankle joint deformities, as well as the degree of knee alignment correction.