Keratocystic odontogenic tumor (KCOT) is a benign intraosseous neoplasm of the jaw which is derived from epithelial remnants of the tooth germ and basal cells of the overlying oral epithelium. The occurrence of KCOT in the maxilla is very less compared to the mandible. The incidence of KCOT in maxillary sinus with impacted third molar is very uncommon. This paper reports such a case of KCOT associated with maxillary third molar involving maxillary sinus, diagnosis and treatment of which was based on thorough radiographic evaluation. Very few cases have been reported so far in literature pertaining to the same.