Inverted papilloma is a rare benign sinonasal tumour, characterized by a potentially invasive nature. The lateral nasal wall represents the most common site of origin, whereas paranasal sinuses are quite frequently found to be involved by extension. In contrast, primary sinus inverted papillomas have rarely been reported. The present study describes an extremely rare case of inverted papilloma, isolated to the left sphenoid sinus, that was treated by a transnasal endoscopic procedure. The therapeutic approach chosen is discussed and the results of a two-year follow-up are also presented.