The plasma cell neoplasms may present in soft tissue as extramedullary plasmacytoma (EMP), in bone as a solitary plasmacytoma of bone (SPB), or as part of the multifocal disseminated disease multiple myeloma (MM). The EMP is rare, comprising around 3% of all plasma cell neoplasm. The majority (80%) occurs in the head and neck region. In this study we report a case of a man, 70 years old, melanoderm, with a lesion of the oral cavity. Upon physical examination, a lesion was found that extended throughout the posterior upper alveolar ridge, as far as the maxillary tuber on the left side, extending towards the palate. Radiographic examination, complementary laboratory exams were performed. Based on the conclusive symptoms of plasmacytoma, the patient was referred to the hematology service for treatment with local radiotherapy. The patient responded satisfactorily to the treatment, and after 15 months, all clinical symptoms of the lesion in the oral cavity had disappeared.