<p>A postoperative intradiploic arachnoid diverticulum secondary to endoscopic third ventriculostomy is an extremely rare occurrence. It has a varied clinical presentation and is caused by the insinuation of the arachnoid through the dural rent into the intradiploic compartment following the procedure. This case report describes an incidentally diagnosed right parietal intradiploic arachnoid diverticulum as a complication of endoscopic third ventriculostomy. Skull radiography revealed an expansion of the right parietal bone with an intact outer table. Non-contrast computed tomography showed a well-defined cystic lesion in the intradiploic compartment, accompanied by expansion of the right parietal bone. A subsequent magnetic resonance imaging distinguished it from an epidermoid cyst. Knowledge of this rare entity is cardinal for early diagnosis and management, required to prevent potential neurological sequelae.</p>