Leiomyosarcoma (LMS) is a rare mesenchymal malignancy, of which 3–10% of cases occur in the head and neck region. We report a 22-year-old woman who was referred to the University Hospital of Siena, Italy, in 2016 with an ostensibly benign asymptomatic lump on the mandibular gingiva. The lesion grew rapidly, causing otalgia in the right ear. An excisional biopsy was performed and primary LMS was diagnosed histologically. Subsequently, the patient underwent radical re-excision of the perilesional mucosa, a partial bone resection and the extraction of four teeth. No recurrences or metastases were detectable at a 20-month follow-up. This report discusses the differential diagnosis of LMS with regards to other benign and malignant lesions and reviews the recent literature on primary and secondary oral LMS. Due to its innocuous clinical features—including its asymptomatic nature and presentation at a young age—this aggressive malignancy can go undetected; therefore, an early histopathological diagnosis is crucial