A 13-year-old, spayed female, Yorkshire terrier was presented with a history of vomiting and diarrhea. Radiographic and ultrasonographic studies suggested rupture of right renal cyst. Left kidney had increased cortical echogenecity with indistinct corticomedullary junction on sonogram. Unilateral nephroureterectomy of right kidney was performed. Histopathologic examination of the resected renal mass confirmed a renal cystadenocarcinoma. To the best of our knowledge, it is the first case report of renal cystadenocarcinoma in a small breed dog in Korea.