Azathioprine is frequently used for the treatment of inflammatory bowel diseases (IBD) such as Crohn's disease and ulcerative colitis. Lymphomas, squamous cell carcinomas, and undifferentiated pleomorphic sarcomas have been reported among patients receiving azathioprine therapy. Herein, we report a case of pleomorphic liposarcoma of chest wall which occurred in a 44-year-old man with Crohn's disease taking azathioprine. He was diagnosed with Crohn's disease 3 years ago after suffering from abdominal pain and hematochezia for 12 years. He had been taking 50 mg of azathioprine per day for 23 months when he visited the thoracic and cardiovascular surgery clinic due to right chest palpable mass that had rapidly grown during the past 2 months. Excisional biopsy was performed and the mass was diagnosed as pleomorphic liposarcoma. Therefore, he underwent radical excision of the right chest wall mass, which measured 11.0 × 6.5 cm in size. He is scheduled to receive radiation therapy and chemotherapy.