Desmoid fibroma is a rare benign tumor of fibrous tissue characterized by a high potential for locoregional invasion and recurrence. The mandibular location of this pathology is exceptional. In our work, we report a case of desmoid tumor observed in a 9-year-old boy. Clinically, he presented a large right cheek swelling that was immovable relative to the mandible. The diagnosis was based on clinical, radiological and pathological arguments. There is no therapeutic protocol for desmoid tumors, but surgery is the most commonly used treatment. Chemotherapy, radiotherapy and hormonotherapy are used in addition to surgery or in some cases of inoperable tumors.