A very uncommon tumor, primary intraosseous carcinoma (PIOC), is a carcinoma arising within the jaw. The definite diagnosis of PIOC is often difficult as the lesion must be distinguished from alveolar carcinoma that may invade the bone from the overlying soft tissues or from the tumors that have metastasized to the jaw from a distant site. A case of PIOC arising in the mandible is presented. The clinical, radiologic, and histologic features are described. This rare lesion should be considered in any differential diagnosis of a jaw radiolucency.