A 16-year-old mare was subjected to examinations due to episodes of nose bleeding, facial bone deformity, dyspnea, and progressive emaciation. Clinical and radiological examinations as well as laboratory analyses revealed the presence of a neoplasia in the paranasal sinuses, which was confirmed by a biopsy examination. The deterioration of the general condition of the animal and the nonoperative nature of the lesion were an indication for euthanasia. The necropsy displayed massive tumor in the bilateral maxillary sinuses, nasal cavity, and hard palate. In addition, nodal metastases of the neck area as well as contact metastases from the affected lymph nodes to the thyroid gland were detected. Based on the histopathological findings and additional immunohistochemical investigations, the tumor was classified as squamous cell carcinoma. This article also provides a review of the literature on various aspects of the presented case.