Superficial temporal artery pseudoaneurysm is rare and usually caused by trauma. Pseudoaneurysms have been reported to occur most frequently from 2 to 6 weeks after trauma and they range from 1 to 3 cm in diameter. The authors present a case of a patient with an atypical delayed rapid growing pseudoaneurysm, who had undergone neurosurgery after head trauma 20 years ago.A 72-year-old woman underwent craniotomy and extradural hemorrhage removal through a left temporoparietal incision caused by head trauma after a traffic accident 20 years prior. The mass of less than 1 cm in diameter was incidentally found by magnetic resonance imaging 8 years ago. However, the patient had no symptoms and the pseudoaneurysm was not considered a significant finding. Several weeks ago, the patient recognized a palpable mass by chance. She observed it without any medical evaluation and treatment. However, the size of the mass suddenly increased without the patient undergoing trauma. It presented as a soft, pulsating round mass of about 3 cm in diameter. Under general anesthesia, the mass was removed without problems. It was a round-shaped mass of 2.2 cm × 2.4 cm in diameter. The transverse cross-section evidenced it was filled with blood clots. The biopsy revealed a dilated vascular wall with an organized thrombus and neovascularization, which are characteristic for a thrombosed pseudoaneurysm.Thus, given that a pseudoaneurysm can grow at any time, medical doctors should strongly consider surgical removal as opposed to simple observation.