We report the case of a 47-year-old woman who underwent a mastoidectomy. Preoperative computed tomography demonstrated an unusually distended bony canal that passed through the superolateral portion of the right petrous bone. Intraoperatively, we identified the anomaly as a petrosquamosal sinus (PSS). This unusually dilated venous channel had arisen from the adjacent sigmoid sinus. A PSS is an emissary vein of the posterior fossa that courses along the petrosquamosal junction, connecting the sigmoid or transverse sinus with the extracranial venous system. While it usually regresses during fetal life, a dilated PSS occasionally persists into adulthood. Its anatomic course may lead to problematic bleeding during mastoidectomy.