Primary intraosseous carcinoma not otherwise specified (PIOC NOS) is a rare tumor assumed to arise from the epithelium, such as odontogenic cysts or benign tumors. Its clinical and imaging diagnoses are often challenging, especially in the early stages, as it mimics jaw cysts and benign tumors, and no specific findings have been identified. This report presents the case of a 66-year-old male patient with mandibular PIOC, highlighting the imaging findings over time. Magnetic resonance imaging (MRI) before symptom onset showed a cystic lesion in the right mandible with a soft tissue component. Both the fluid component and soft tissue exhibited low apparent diffusion coefficient values (1.0 × 10