Clinical and pathological findings in a case of malignant tumor located at nasal and paranasal sinuses in a six year old donkey is reported. The animal was referred to the Equine Veterinary Hospital after three years of progression. Clinically, the animal presented tachypnea, dyspnea and wheezing at the laryngotracheal region. At the frontal bone and at the left orbitary region there was swelling with exophtalmos of the left globe and bilateral mucopurulent nasal discharge, as well as retropharyngeal lymph node enlargement. Radiographic examination revealed radiopacity in the left maxilar, frontal and nasal sinuses and in the left orbitary region. Endoscopy revealed an irregular mass in the nasopharynx. Computed tomographic images of the skull showed an irregular mass, with variable density, in the rostral region extending from the nasal cavity to the conchofrontal sinus, at the left side, with dorsal displacement of the septum and dorsal nasal meatus. Loss of definition and destruction of other osseous structures of the nasal cavity and the frontal bone, and hypodense areas at the encephalic periphery were evident. Computed tomographic scans were taken to define the localization and extension of the mass. Fibrosarcoma was confirmed for the the first time in this species after histopathological examination. Tomographic scan was considered more effective than radiography.