A 7-year-old castrated male Persian cat presented with a cutaneous mass and an increase in serum amyloid A concentration. Fine needle aspirates of the mass indicated lymphoma, which was also the top differential diagnosis on histopathologic examinations. Immunohistochemically, neoplastic cells tested negative for anti-CD3, PAX5, CD20, and c-Kit, but positive for MUM1, CD79α, and CD138, suggesting extramedullary plasmacytoma. There were tumor-infiltrating non-neoplastic CD3+ T and CD20+ B cells. Practitioners should be aware of feline plasmacytoma characterized by lymphoma-like cytologic and histologic features. The present study is valuable in providing the first clinical evidence that proves the immunogenicity of feline plasmacytoma.