Primary liposarcomas of the mediastinum are unusual tumors. We report herein a case of a 52-year-old woman, who was found to have a mediastinal tumor involving both hemithoraces and radiologically showing non-resectable-invasive features to the adjacent vital structures. She had a history of left thoracotomy for mediastinal schwannoma 14 years previously. The patient underwent an exploratory thoracotomy following a preoperative misdiagnosis of an ancient schwannoma. Complete removal of the tumor was accomplished through a right posterolateral thoracotomy with a subsequent histological diagnosis of a recurrent low-grade liposarcoma. A resectable liposarcoma should be considered in the differential diagnosis of a mediastinal tumor, although radiologically, the tumor presents with invasive features.