Plasmacytomas are a localized proliferation of plasma cells in the bone marrow and soft tissue. Extramedullary plasmacytomas are rare and typically solitary plasma cell neoplasms originating from extraosseous organs and tissues. A 31-year-old woman was referred to our hospital with a rapidly growing abnormal shadow on a chest roentgenogram. Chest computed tomography showed that the lesion was located in the anterior mediastinum. She underwent surgery, and the tumor was diagnosed as an extramedullary plasmacytoma. She remains well 2 years postoperatively without recurrence. An extremely rare case of an anterior mediastinal extramedullary plasmacytoma is presented.