Rupture of a renal artery aneurysm is an acute surgical event associated with high mortality. We report a case of retroperitoneal hemorrhage from a spontaneously ruptured renal artery aneurysm. A 73-year-old woman complained of left flank and abdominal pain. She consulted our department and left retroperitoneal hemorrhage was recognized by abdominal computerized tomography. Selective left renal arteriography revealed a saccular aneurysm arising from the ventral branch of the renal artery, and did not show extravasation of contrast material from the aneurysm. Since it was difficult to remove the aneurysm with preservation of the involved renal unit, we performed left nephrectomy.