Osteosarcoma is a malignant mesenchymal tumor whose cancerous cells produce osteoid matrix. It is the most common primary malignant bone tumor, accounting for approximately 20% of the sarcomas, but only 5% of the osteosarcomas occur in the jaws. They present various clinical and histological aspects, as well as variable disease progression and outcome. This article shows a case report of a 20-year-old woman who presented swelling near the mandibular left premolar. After clinical diagnosis of lesion of the dental periapex, the patient initially underwent endodontic treatment of the tooth involved. Thereafter, in a period of eleven days, a significant increase of the lesion could be observed, resulting in visible facial asymmetry. The occlusal radiographic view showed an area of bone destruction and abnormal bone formation in the region. The external cortical portion showed clear radiopacity resembling sunrays, suggesting the diagnosis of osteosarcoma. The treatment comprised partial mandibulectomy and reconstruction of the area, using bone of the rib and skin graft from the buttock for the oral mucosa involved. Eight months after surgery, there was local recurrence of the lesion and the patient died approximately one year after relapse.