Background Primary intraosseous squamous cell carcinoma is a rare malignant central jaw tumor derived from odontogenic epithelial remnants. It predominantly affects the mandible, although both jawbones may be involved. Objective(s) The purpose of this article was to represent the clinical, radiologic, and microscopic characteristics of a late diagnosed primary interosseous squamous cell carcinoma (PIOSCC). This case adds to the very few cases of PIOSCC in the literature. Study Design This case report describes a 60-year-old man who was initially misdiagnosed with a periapical infection related to the right lower wisdom tooth, that is, an infected odontogenic cyst of the mandible. After 4 months, the patient presented to a private dental clinic with a large swelling at the right side of the mandible. Results Extraoral examination revealed a swelling with paresthesia in the right mandibular angle. A panoramic radiograph was requested and revealed a complete erosion of the right ramus with an extension of the lesion to the body of the mandible. Advanced imaging revealed a large, ill-defined, right facial soft tissue mass lesion measuring 8.8 × 4.5 × 5 cm. The microscopic examination revealed fibrofatty tissue infiltrated by a tumor mass formed of solid groups of large pleomorphic atypical squamous cells with high nuclear-to-cytoplasmic ratio and focal intracytoplasmic keratin formation. The diagnosis was PIOSCC. Discussion/Conclusions Radiologic and clinical characteristics of PIOSCCs are similar to those of benign odontogenic cysts and tumors. In certain cases, early-stage PIOSCC may mimic routine dental disorders, such as periapical and periodontal disease, which may lead to misdiagnosis or delayed diagnosis. Clinicians must be aware that PIOSCC may initially present as a routine dental disorder, and that unsuccessful treatment may be a sign of underlying malignancy.