Odontogenic keratocyst is an aggressive cystic lesion and a common type of tooth derived cyst due to presence of odontogenic epithelial remnants in different regions of jaw. In majority of cases, it is located in mandibular posterior region. But it can also be found in the maxilla especially in the canine region. We present a rare case of OKC in maxillary sinus which associated with ectopic third molar. Also, it can be easily confused with other lesions of maxillary sinus like sinusitis or antral polyps, which usually resemble symptomatically. There can be malignant transformation of this benign condition towards squamous cell carcinoma or ameloblastoma. So an early and accurate diagnosis of odontogenic keratocyst is a challenge for pathologists.