Plasmacytomas are a localized proliferation of plasma cells in the bone marrow, and are less frequently seen in extraosseous organs or tissues. Extramedullary plasmacytoma is a rare malignant neoplasm, and is especially uncommon when it arises from the mediastinum. Here, we report on a case of posterior mediastinal extramedullary plasmacytoma in a 64-year-old man. He was admitted with an asymptomatic right apical mediastinal mass, which was provisionally diagnosed as a neurogenic mass. However, a subsequent investigation revealed that this tumor was a rare case of IgG kappa type extramedullary plasmacytoma arising from the posterior mediastinum. The patient was treated with local radiation to the mediastinum and is doing well without further evidence of disease.