A 4.5-year-old female degu (Octodon degus) was minimally responsive with a poor body condition, a rough haircoat, and moderate dehydration. Blood was present around its urethral orifice and on the cage bedding. Laboratory analyses revealed leukocytosis with neutrophilia and anemia; hypoproteinemia and hypoalbuminemia; hyperglycemia, hyperphosphatemia, and elevated alanine aminotransferase, blood urea nitrogen, and creatinine; and hematuria and pyuria with occasional squamous and transitional epithelial cells. A urine culture was positive for coagulase-negative Staphylococcus sp. On gross necropsy, the right kidney was enlarged, cystic, and greenish-brown, with a 10-mm, hemorrhagic, granular mass extending from the renal pelvis into the cranial cortex. Only a small amount of renal cortex appeared normal. The urinary bladder had focal areas of hemorrhage and contained frank blood. Histologically, the papillary mass in the right renal pelvis comprised basophilic, moderately anaplastic, clustered epithelial transition cells consistent with a transitional cell carcinoma. Internally, the tumor showed squamous metaplasia and moderate multifocal interstitial fibrosis. The right kidney cortex contained a choristoma comprising trabecular bone, mature adipocytes, and cellular infiltrates suggestive of osteocytes, lymphocytes, and plasma cells. The urinary bladder had mild to moderate, focal, hemorrhage with neutrophilic inflammation and contained focal areas of mild transitional cell epithelial hyperplasia; these changes may have been secondary to irritation by hemorrhage in the renal pelvis. There was no evidence of metastasis. Renal transitional cell tumors are rare in rodents. This is the first report of both a renal transitional cell carcinoma and a renal choristoma in a degu.