We report here a case of massive nasal bleeding from the sphenopalatine artery three weeks after endonasal transsphenoidal surgery. This 66-year-old male suffered from massive nasal bleeding with the status of hypovolemic shock. Under general anesthesia, an emergent angiography revealed an extravasation from the sphenopalatine artery. Trans-arterial embolization using coil and n-butyl-cyanoacrylate (NBCA) was performed following the diagnostic angiography. Complete occlusion of the injured artery was achieved. The patient showed good recovery from general anesthesia. Delayed nasal bleeding after endonasal transsphenoidal surgery is a rare but important complication. The sphenopalatine artery and its branch are located in the hidden inferior lateral corner of the sphenoid sinus and may be injured during enlargement of the sphenoid opening. When massive delayed nasal bleeding follows transsphenoidal surgery and damage of the internal carotid artery has been ruled out, endovascular treatment of the external carotid artery should be considered.