A 66-year-old female patient had a firm, non-tender, dome shaped mass on the scalp. The lesion had enlarged slowly for 2 years, and measured about 4 × 6 cm. The histologic finding of the skin biopsy specimen demonstrated an infiltration of immature plasma cells in the dermis, which express monoclonal cytoplasmic lambda light chain by immunohistochemical stainings, and staging work-up after the biopsy revealed no evidence of disease in other foci. The mass on the scalp was treated successfully by radiation therapy, with the diagnosis of primary cutaneous plasmacytoma.