Leiomyosarcoma (LMS) of the oral cavity, a rare mesenchymal tumor exhibiting smooth-muscle differentiation, is extremely uncommon in childhood. The most frequent location of childhood LMS is the gastrointestinal tract, particularly the stomach. The purpose of this paper is to report a case of leiomyosarcoma affecting the gingival tissues and mandible of a 9-year-old girl with peculiar clinical, microscopic, and radiographic features. Clinical and radiographical examinations revealed a gingival growth affecting the primary mandibular right first molar with inflammatory features. The lesion was initially suspected to be pyogenic granuloma and was removed by excisional biopsy. Microscopic findings showed a hypercellular proliferation of mesenchymal spindle cells, suggesting malignant spindle cell neoplasm. Immunohistochemical, histochemical, and radiographic studies were undertaken, and the final diagnosis established was a low-grade leiomyosarcoma in the gingiva.