Plasmacytoma is a malignant tumor consisting of neoplastic proliferation of plasma cells, and it is extremely rare to be isolated in the jaw. We treated a patient with solitary plasmacytoma in the mandible.A 42-year-old woman was referred to our hospital for further investigation of an osteolytic lesion in the mandible.A 16×9 mm radiolucent area in the right mandibular ramus was found on panoramic radiography. The osteolytic lesion had expanded compared to panoramic radiography 6 years earlier. The patient underwent an excisional biopsy under general anesthesia. Histopathological examination confirmed the diagnosis of plasmacytoma. As there were no abnormal findings in whole body bone, bone marrow aspiration, blood test, or urine test, she was diagnosed with solitary plasmacytoma of the mandible. The patient underwent local radiotherapy with a total dose of 45Gy. To date, 41 months after radiotherapy, there have been no signs of recurrence or progression to multiple myeloma.