We report the case of a 79-year-old man for whom investigations of superior vena cava syndrome have revealed extramedullary plasmocytoma. Extramedullary plasmocytoma (EMP) is the less frequent form of plasma cell neoplasms. The mediastinal location of EMP is rare, and its compressive nature is exceptional. Another particular feature we observed is that the EMP was associated with an authentic kappa light chain multiple myeloma.