Leiomyosarcomas (LMSs) are rare malignant mesenchymal tumors which show smooth-muscle differentiation. Most LMSs involving the oral tissues primarily affect the maxillary sinus, the maxillary or mandibular bone. We present a case of LMS of the mandibular alveolar mucosa, arising in a 12-year-old male child from Ethiopia. A malignant spindle cell-like neoplasm was diagnosed on clinical and radiographic findings as well as on incisional biopsy. The tumor was resected with wide margins. The following histopathologic examination with additional immunohistochemical studies secured the diagnosis LMS. Microscopically, the spindle-shaped tumor cells were arranged in an interlacing fascicular pattern and contained oval to elongated, blunt-ended (cigar-shaped) nuclei. The immunohistochemical examination showed immunoreactive tumor cells for vimentin, actin, desmin, and H-caldesmon, which is pathognomonic for LMS. Immunohistochemical studies are mandatory to differentiate the LMS from other similar spindle cell neoplastic lesions. Radical resection with safety margins and a lifelong periodic follow-up has to be recommended.