Squamous cell carcinoma (SCC) derived from keratocystic odontogenic tumor is an extremely rare tumor that is limited to the jaws. Most intraosseous carcinomas originate from the epithelial lining of odontogenic cysts, so they are called odontogenic carcinomas. They occur more frequently in men and the mean age of patients is 57 years. The following report describes an extremely rare case of an odontogenic carcinoma derived from a keratocystic odontogenic tumor in a 20-year-old man. The patient presented with an exophytic lesion in the retromolar region of the mandible which was first noticed by the patient 25 days earlier. In panoramic radiograph a well-defined radiolucency around the impacted 3rd molar was observed. Clinical and radiographic diagnosis was odontogenic SCC. Surgical resection was performed and histhopathologic examination of the lesion confirmed the diagnosis of SCC in the wall of a keratocystic odontogenic tumor.