Superficial temporal artery aneurysms are rare; when they do occur, they are usually associated with head trauma. Spontaneous true aneurysms of the superficial temporal artery are extremely rare. They are classified as true aneurysms when all three layers of the vessel are found to be involved on histologic examination. Therapeutic options include conservative management, image-guided embolization, and surgical excision. We report a case of an extracranial spontaneous aneurysm of the frontal branch of the superficial temporal artery. A 20-year-old man presented with an asymptomatic, pulsatile, 1-cm forehead mass that had gradually increased in size. The aneurysm was evaluated by clinical examination and three-dimensional computed tomographic angiography. Complete resection was performed with local anesthesia. Histologic examination revealed that the aneurysm involved all three layers of the blood vessel: the tunica intima, tunica media, and tunica adventitia. No atherosclerotic changes or inflammatory cells were found. To the best of our knowledge, this is only the third reported case of a histologically verified spontaneous aneurysm of the frontal branch of the superficial temporal artery. Awareness of this rare pathology in the differential diagnosis of a forehead mass may facilitate diagnosis and prevent complications.