A 65-year-old female visited our hospital complaining of a swelling on the left cheek area of 2 years' duration. A panoramic radiograph revealed an ill-defined osteolytic radiolucent bony lesion involving the left mandibular angle, ascending ramus, coronoid process and condylar process. Histological examination showed the mandibular lesion to be a plasmacytoma, and a systemic work-up was obtained to rule out multiple myeloma. Contrast-enhanced CT images showed a well-defined and slightly enhanced round mass on the left ramal area, accompanied by the destruction of the left ramus and posterior maxilla. An (18)F-fluorodeoxy-glucose positron emission tomography CT ((18)F-FDG PET/CT) scan revealed a hypermetabolic mass extending from the left mandible to the left maxillary sinus. The patient had M-protein in serum and urine, plasma cells up to 36.5% on bone marrow biopsy and anaemia as a clinical complication. The patient was diagnosed with multiple myeloma and received chemotherapy with thalidomide, cyclophosphamide and dexamethasone. A PET/CT scan taken 6 months later revealed that the hypermetabolic mass had disappeared and there was remarkable bone formation on the left mandible compared with a previous PET/CT scan. A panoramic radiograph taken 8 months later also demonstrated a prominent bone formation of the affected site. To the best of our knowledge, the current case is the first report of multiple myeloma presenting as plasmacytoma of the mandible with an FDG PET/CT scan. The lesion was solitary at diagnosis, and remarkable bone formation was newly observed on the radiographic examination during chemotherapy.