A 5-year-old mixed-breed dog presenting with diffuse lesions was diagnosed with cutaneous plasmacytoma. Sites where tumors occurred most often involved the skin of the facial areas, as well as that of the shoulder, neck, and back of the body. No systemic findings were present in spite of a significant enlargement of the lesions. The tumors varied in diameter, the largest being 5-10 cm. Tissue samples obtained from tumoral masses were fixed in 10% formalin, processed routinely, embedded in paraffin, cut into sections of 4-5 mu m, and stained with hematoxylin and eosin. To determine the nature of the tumoral cells, kappa and lambda light chain antibodies were applied to the paraffin sections immunohistochemically. The tumor was diagnosed as cutaneous plasmacytoma according to its morphological and microscopical properties. Tumor cells displayed features of asynchronous plasmacytoma. The patient was cured by medical treatment. The disease and the chemotherapy procedure ended with a favorable prognosis and are discussed here due to the rarity of such cases.