Primary dedifferentiated liposarcoma originating in the mediastinum is extremely uncommon. Here, we report a case of mediastinal dedifferentiated liposarcoma. A 74-year-old man was admitted to our hospital with a left intrathoracic tumor ∼10 cm in diameter. Computed tomography that was taken in another hospital 2 years before the initial visit to our hospital had shown a lipomatous tumor ∼3 cm in diameter adjacent to the left lower pulmonary vein. The tumor was resected by left lateral thoracotomy with video-assisted thoracoscopic surgery. Pathological examination revealed primary mediastinal dedifferentiated liposarcoma. No evidence of recurrence has been seen as of 8 months postoperatively.