We examined a female Kiso horse (a native Japanese horse breed) aged 2 years and 9 months with persisting nasal discharge that died due to lymphoma. Necropsy revealed masses in the left nasal cavity, left periorbital cavity, and left maxillary and mandibular areas. Histologically, the neoplastic tissues were composed of lymphoid and plasmacytoid cells, with immunoglobulin A (α and λ chains) production in the latter. The foci of large lymphoid cells were detected at some sites. All neoplastic cell types were positively stained in immunohistochemical staining for CD20. Extranodal marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma) was suspected on the basis of the presence of lymphoepithelial lesions, plasmacytic differentiation, and large-cell transformation. A history of chronic rhinitis may be linked to the development of this disorder.