Primary liposarcoma of the pectoral major muscle is extremely rare. We report a case of liposarcoma of the pectoral major muscle which was treated with surgical excision and postoperative radiotherapy. A 70-year-old man admitted with left-sided painless progressively growing breast mass. Radiological investigation revealed liposarcoma of the pectoralis major muscle. The patient was treated by surgical removal. Pathological diagnosis was pleomorphic liposarcoma. The patient had postoperative radiotherapy and free of disease for 9 months. Surgical excision and postoperative radiotherapy is the most favored treatment strategy. Careful follow-up is mandatory for detecting recurrences.