A rare case of extramedullary plasmacytoma in the mediastinum is reported. An 80-year-old man was admitted for further examination of a mediastinal tumor. Chest computed tomography (CT) revealed a large mediastinal mass and right interlobar pleural effusion. Needle biopsy under CT guidance established a diagnosis of plasmacytoma. Immunohistochemical staining revealed that the tumor cells were producing monoclonal IgA kappa. Serum immunoelectrophoresis revealed an IgA kappa monoclonal component with a serum concentration of 5,040 mg/ml. The bone marrow aspiration was normal. Bone roentgenogram and bone scintigram showed osteoporosis but no neoplastic lesion.