Leiomyosarcoma is a malignant neoplasm characterized by proliferation of cells with smooth muscle differentiation. It is extremely rare in the oral cavity; to date, only well-documented 29 cases of primary oral leiomyosarcoma have been reported. A 35-year-old man was referred complaining of a painful tumor mass on the right mandibular body, lasting for 45 days. During anamnesis, the patient reported previous dental examination, being diagnosed as pericoronitis, followed by antibiotic prescription, but the lesion continued to increase. Extra-oral examination showed prominent facial asymmetry on the right side. Intra-oral examination revealed an extensive tumor mass measuring 8 × 5 × 2 cm on the right mandibular region. Panoramic radiography showed an ill-defined radiolucent area. After incisional biopsy, microscopy revealed a spindle cell malignant neoplasm. Immunohistochemistry showed positivity for vimentin, alpha-smooth muscle actin, and focally for desmin. Ki-67 was 50%. A diagnosis of primary oral leiomyosarcoma was established. The patient was referred for oncological treatment.