Plasmacytoma are extramedullary accumulations of plasma cells. Most extramedullary Plasmacytomas are associated with the upper respiratory tract. The mediastinum is rarely involved. We report a rare case of mediastinal plasmacytoma with multiple myeloma. The patient is 66 year old woman presented with bone pains and mediastinal mass on CT scan and MRI. The preliminary diagnosis of occult lung cancer with mediastinal involvement, and widespread skeletal metastasis was made, although lymphoproliferative disorder along with germ cell tumor was also kept in differentials.The diagnosis of mediastinal plasmacytoma with multiple myeloma was made after extensive investigations.