We report herein a case of renal capsular hemangiosarcoma. A 68-year-old man was admitted to our hospital for treatment of a retroperitoneal tumor identified incidentally on abdominal computed tomography (CT) for follow-up of superficial bladder tumor. The tumor was about 7cm in diameter, positioned between the right kidney and the liver. Right nephrectomy was performed under a diagnosis of renal capsular tumor. Pathological diagnosis was hemangiosarcoma and positive surgical margins were suspected. Radiotherapy was performed postoperatively to a total dose of 50 Gy. Hemangiosarcoma frequently occurs in the skin, but is rare in the retroperitoneal cavity. Neither metastasis nor recurrence has been seen as of 19 months postoperatively.