Sinonasal inverted papilloma is an expansive, benign mass derived from the Schneiderian membrane. It may undergo malignant transformation and most commonly originates from the maxillary sinuses or the lateral walls of the nasal corridors. This case outlines the case of a sinonasal inverted papilloma, which clearly arises from the posterior ethmoid sinus, bordering the skull base. This abnormal originating point was able to be identified during endoscopic excision of the mass and involved mucosa. The ethmoid bone was not resected as it would expose the dura mater, risking CSF leak and complications. This case further supports the use of endoscopy in the investigation of sinonasal inverted papilloma rather than the gold standard approach of lateral rhinotomy. An endoscopic approach allowed for improved safety when accessing the posterior ethmoid cavity. This case also highlights the possibility of novel origins of sinonasal inverted papilloma, such as the membrane of the posterior ethmoid cavity.