A 62-year-old woman presented with simultaneous subarachnoid hemorrhage (SAH) and massive epistaxis. The patient had been treated for pituitary prolactinoma by two transsphenoidal surgeries, gamma knife radiosurgery, and conventional radiation therapy since age 43 years. Cerebral angiography showed left petrous internal carotid artery (ICA) aneurysm with slight stenosis on the adjacent left petrous ICA. She underwent superficial temporal artery-middle cerebral artery (STA-MCA) double anastomosis with endovascular internal trapping without complication the day after onset. Postoperative course was uneventful; the patient did not develop symptomatic vasospasm, recurrent epistaxis, or cerebrospinal fluid rhinorrhea. Postoperative angiography demonstrated complete disappearance of the aneurysm with patent STA-MCA anastomosis. The patient was discharged 2 months after surgery without neurological deficit. The present case is extremely rare with simultaneous onset of SAH and epistaxis caused by ruptured petrous ICA aneurysm. The transsphenoidal surgeries and radiation therapies might have been critical in the formation of the petrous ICA aneurysm.