A 13-year-old domestic shorthair cat was presented with weight loss and azotemia. Abdominal ultrasound revealed a large cystic space- occupying lesion with multiple septae in the left kidney. A core needle biopsy yielded a renal cystadenoma originating from the epithelial cells. This report describes the clinical, ultrasonographic and computed tomographic features and the growth progression of a renal cystadenoma. We describe the first attempt to apply the human Bosniak classification to a cat with renal cystic neoplasia to differentiate between benign and malignant lesions. Cystadenoma should be a differential diagnosis in cases of renal cystic space-occupying lesions. Other differentials, imaging features to differentiate benign and malignant lesions and the risk of malignant transformation will be discussed.