Extramedullary plasmacytoma (EMP) is a plasma cell neoplasm without bone marrow involvement or other systemic characteristics of multiple myeloma. Few large-scale clinical studies have been conducted because of the rarity of EMP, especially when it arises from the mediastinum. Herein we report a rare case of solitary mediastinal plasmacytoma with reactive pleural effusion. A 58-year-old female presented with grade 4 dyspnea and dysphagia, with a mediastinal mass observed with on PET. CT-guided biopsy results were suggestive of IgG kappa-type EMP arising from the anterior mediastinum. The patient was treated with local radiotherapy to the mediastinum, and had clinical and radiological response s were good. Radiotherapy is an effective treatment for mediastinal EMP, but a complete workup is mandatory, including PET, as the majority of such masses coexist with multiple myeloma.