The authors present the case of an elderly patient with a quadrigeminal arachnoid cyst who was successfully treated with endoscopic fenestration through the posterior wall of the third ventricle via the anterior horn of the lateral ventricle. This 71-year-old man suffered from progressive gait instability and disorientation. Radiological examination revealed hydrocephalus caused by a quadrigeminal arachnoid cyst. The patient underwent endoscopic fenestration of the quadrigeminal cistern arachnoid cyst and third ventriculostomy via one burr hole placed at the coronal suture. This method is less invasive and is effective for quadrigeminal cistern arachnoid cyst and accompanying hydrocephalus.