A 69-year-old man was pointed out the anterior mediastinal mass on chest computed tomography (CT). Since the thymoma was suspected, thymectomy was performed through median sternotomy. The tumor was encapsulated and did not directly invade adjacent organs. The tumor was 8.3×5.3×8.2 cm in size. Histological examination of the resected specimen showed a large mucinous component and a small fibrous component, both containing many atypical spindle cells and multinuclear giant cells, and fat cells and atypical lipoblasts observed in a small part of the specimen. The postoperative pathological diagnosis was dedifferentiated liposarcoma. No relapse is observed without adjuvant therapy at 1 year after surgery.