Objective. We report the first utilisation of transnasal marsupialization to treat a keratocystic odontogenic tumor in the maxillary sinus of a 37-year-old man. Case Report. A 37-year-old man presented with a nasal discharge and right odontalgia. Computed tomography revealed an expanding cystic lesion with a calcificated wall containing an impacted tooth in the right maxillary sinus. The diagnosis was keratocystic odontogenic tumor. Transnasal marsupialization was performed using endoscopic sinus surgery to enlarge the maxillary ostium and remove a portion of the cystic wall. Pathological findings included lining squamous epithelium and inflammation. The remaining tumor shrank, becoming free of infection after surgery, without proliferation. Conclusion. Transnasal marsupialization using endoscopic sinus surgery is effective in treating keratocystic odontogenic tumors. It offers minimal surgical invasion and reductive change, making it advantageous for complete removal with fewer complications in the bones and surrounding tissue in the case of secondary surgery.