Primary intraosseous carcinoma (PIOC) is a rare squamous cell carcinoma (SCC) arising within the jaw, which has no initial connection with the oral mucosa, overlying skin, antral or nasal mucosa. Primary intraosseous carcinoma is also designated as odontogenic carcinoma because it is considered to develop from the epithelium involved in odontogenesis. It is considered a rare lesion, but may not be as rare as commonly believed. It may arise de novo or as a consequence of malignant transformation of a benign cyst or tumor. It is locally aggressive with a poor prognosis. A case of primary intraosseous SCC of the mandible, with evidence of origin in an odontogenic cyst, is presented.