Multiple extramedullary (soft tissue) plasmacytoma is a rare neoplasm of plasma cells. This is a case of a 63-year-old male who presented with back pain and a one month history of a painful, progressively growing mass at the right anterior chest wall. Since radiographs and computed tomography of the chest showed lytic lesions on the ribs and L4 compression deformity, the patient was referred for whole body bone scintigraphy to evaluate the remainder of the skeleton. Bone scintigraphy revealed osteoblastic lesions and osteolytic lesions in the thoracic cage. Diagnosis of plasmacytoma was established by biopsy and confirmation of microscopic characteristics and immunohistochemical staining.