Primary intraosseous carcinoma (PIOC) of the jaw is a rare neoplasm arising from the lining epithelium of odontogenic cysts or de novo from odontogenic epithelial rests that has no communication with the surrounding mucosa of the upper aerodigestive tract. We present a case of PIOC ex-odontogenic keratocyst (PIOC ex-OKC) in a 35-year-old male. Histopathologic examination revealed a cystic lesion with a fibrous capsule lined by corrugated parakeratinized stratified squamous epithelium resting on a basal cell layer composed of columnar cells exhibiting palisaded hyperchromatic nuclei, features consistent with OKC. Surgical treatment consisted of bilateral crestal and crevicular incision, a reflection of the flap, breaking of all OKC locules, creation of a continuous cavity, and fitting of a decompression mold around the mandibular teeth. This case highlights the importance of knowing the features of PIOC and considering PIOC in the differential diagnosis of malignant tumors of odontogenic epithelium for timely surgical treatment.