Odontogenic Keratocyst (OKC) is a clinico-pathologically distinct form of developmental odontogenic cyst. It is known for its aggressive biological behavior and high recurrence rate. Odontogenic keratocyst (OKC) is the third most common odontogenic cyst and comprises about 12% of all the cysts occurring in the maxillofacial region. OKC has a distinct feature to occur in body of the mandible and ramus. But it can also be found in the maxilla especially in the canine region. We present a rare case of OKC in maxillary sinus of a 35 year old female patient which is associated with ectopic third molar that can be easily confused with other lesions of maxillary sinus like sinusitis or antral polyps. Involvement of the maxillary sinus by OKC is rare with < 1% of cases reported in the literature.