Extramedullary plasmacytoma (EMP) is a rare soft tissue plasma cell tumour, usually presenting as a mass within the upper respiratory tract. It is most unusual for it to present in the posterior mediastinum as a primary solitary lesion. Discussed here is the case report of a patient who initially presented with musculoskeletal pain. Subsequent investigations, however, revealed the cause to be an EMP; diagnosis was made on finding aggregates of plasma cells on histological examination of the tissue specimen. The latter had been obtained by way of a computerised tomography scan guided biopsy of the tumour. Bone marrow aspiration studies showed no evidence of multiple myelomatosis. There was significant clinical improvement in response to radiotherapy. It is possible that this might represent one of the first documented reports of a primary solitary EMP in the posterior mediastinum.