Introduction and importance: Synovial lipomatosis is a rare disease that often results in joint discomfort, swelling, and effusion. Only a few reported cases have been documented so far. Case presentation: We reported a 50-year-old woman with synovial lipomatosis of the ankle joint. The patient presented with left ankle pain since the least two years, accompanied by swelling and redness. Physical examination demonstrated swollen ankle with warmth palpation. Magnetic resonance imaging (MRI) demonstrated hyperintensity of the ankle joint on the posterior and anterolateral side, thickening of the synovium, and bone marrow edema around the talar bone, surrounding the sinus tarsi. Histopathological examination demonstrated fatty tissues with nonspecific inflammation, suggesting synovial lipomatosis. We performed synovectomy, and the pain did not recur at 1 year of follow-up. Clinical discussion: Synovial lipomatosis is a highly uncommon benign condition. Histopathologically, synovial lipomatosis is characterized by notable adipocyte infiltration into the subsynovial tissue and a villous or frond-like morphology of the synovium. It also features transversely hyperplastic synovial lining cells. Conclusion: Synovial lipomatosis of the ankle joint is a rare entity. Further studies are required to investigate this disease and its management. © 2024 The Authors