Primary intraosseous odontogenic squamous cell carcinoma (PIOSCC) is a rare tumor. The incidence is low, and approximately 200 cases are reported in literature. The etiology is associated with the malignant degeneration of embryological remains. Differential diagnosis includes alveolar carcinomas, jaw metastases from other locations, odontogenic tumors, and tumors of the maxillary sinus. However, the diagnosis could be delayed due to the absence of symptoms in early stages. Surgery represents the first choice treatment. Whereas, postoperative radiotherapy could be helpful to improve the overall survival. The prognosis is generally poor. In this report, we describe the case of a 77-year-old man accidentally diagnosed PIOSCC. The patient referred a tooth extraction and cystectomy 3 years before. The pathological examination of the cyst tissue evidenced an inflammatory cyst. However, no patient follow-up was performed. Hence, is important to stress that routine monitoring of patients affected by large inflammatory cysts of the jaw bones should be mandatory.