An undifferentiated renal tubular carcinoma was diagnosed in a juvenile male olive baboon (Papio anubis). The animal suddenly appeared depressed and refused to eat. During physical examination, a firm, palpable mass in the left abdominal area and flank pain were detected. Clinical pathology findings included mild anemia, hypoalbuminemia, hyponatremia, and mildly increased serum creatinine and urea concentrations. Radiographs revealed a large mass in the left abdominal area. Exploratory laparotomy disclosed a 10 cmx15 cm multilobulated mass involving the left kidney and adjacent organs. Because of a poor prognosis, the animal was humanely euthanized, and necropsy was performed. Tissue samples of the neoplasm were taken for histopathological examination. Immunohistochemical staining was done using vimentin, cytokeratin, S-100 protein, Ki-67, alpha-actin, and desmin-specific primary antibodies. Microscopically, elongated and irregular tubules were lined by 2 or more layers of atypical epithelial cells. Anisocytosis, anisokaryosis, and frequent mitotic figures were also observed. Following immunohistochemical staining, the cytoplasm of neoplastic cells was positive for cytokeratin, vimentin, and S-100 protein and negative for alpha-actin and desmin. Positive nuclear staining for Ki-67 was observed. The neoplasm was diagnosed as an undifferentiated renal tubular carcinoma.