Odontogenic keratocyst is a common and aggressive cystic lesion derived from tooth remnants in the jaw. It is frequently found in the posterior mandible and less common in the maxilla. This report presents a rare case of an odontogenic keratocyst in maxillary sinus which associated with ectopic third molar. Due to its symptomatic resemblance to other maxillary sinus lesions such as sinusitis or antral polyp, pathologists often face challenges in accurately and promptly diagnosing odontogenic keratocyst.