A 35-year-old male showed slow progression of dilation of the lateral ventricles and third ventricle. He had undergone surgery for third ventricular colloid cyst and ventriculoperitoneal shunting when he was 3 years old. Computed tomography revealed progression of triventricular dilation. He underwent endoscopic fenestration of the arachnoid cyst and endoscopic third ventriculostomy. The postoperative course was good. Arachnoid cysts within the third ventricle are rare. Endoscopic treatment of an arachnoid cyst within the third ventricle is less invasive and effective for arachnoid cysts within the third ventricle associated with hydrocephalus.