This is a concise paper which describes a previously undefined type of cell within the sphenoethmoidal complex. It identifies the retrosphenoid cell, differentiated from an Onodi cell by being entirely within the posterior wall of the sphenoid sinus, lying between the sphenoid and the clivus. This can be identified on a preoperative CT scan, and is important to identify as failure to drain disease in this cell can result in inadequate surgery and the need for a revision procedure. It is a nice reminder of the importance of careful and detailed evaluation of the preoperative CT to ensure adequate surgery to reverse the patient’s symptoms and to avoid any complications.