We report a case of unicystic ameloblastoma associated with an ectopic third molar in the right maxillary sinus, which was misdiagnosed as a dentigerous cyst on preoperative small incisional biopsy. Surgical enucleation of the cystic lesion was performed under general anesthesia with immediate reconstruction of the maxillary sinus using titanium mesh plate. The patient's postoperative recovery was uneventful, and there was no evidence of tumor recurrence during the 7-month follow-up period.