A 97-yr-old, male, captive desert tortoise (Gopherus agassizii) had a 2-month history of lethargy. Imaging with ultrasound, radiographs, and computed tomography revealed a 10-cm-diameter mass in the caudal coelom. Fine-needle aspiration revealed spindle and epithelial cell proliferations with formation of rosettes. Exploratory surgery was performed and the mass was removed and submitted for pathology. There was no evidence of metastasis on imaging or at surgery. Histology revealed a mass arising from and compressing the kidney. The mass was composed of primitive glomeruli, tubules, blastemal cells, and mesenchymal cells, features that are diagnostic for nephroblastoma. Tubules were reactive to cytokeratin and mesenchymal cells were reactive to desmin via immunohistochemistry; other immunohistochemical markers were either negative (e.g., S-100) or noncontributory (e.g., epithelial membrane antigen, myogenin, vimentin, and Wilms' Tumor 1). This is the first report of nephroblastoma in a chelonian.