Background: Ewing sarcoma (ES), a rare malignancy, comprises whatever the age, 4–15% of all primary bone tumors. It represents 1% of all malignant tumors in children and is the fourth most common bone malignancy after myeloma, osteosarcoma, and chondrosarcoma. Case description: A 12-year-old boy came to the Oral Surgery Department of Bretonneau Hospital referred by his dentist with a rapidly evolving swelling in the left mandibula for 6 weeks, which was initially diagnosed as a facial cellulitis. Cone beam computed tomography (CBCT) showed a poorly defined, expansile, and osteolytic tumor on the left side of the mandible. Clinical and radiographic findings were in favor of an aggressive primitive bone tumor. A mandibular biopsy under general anesthesia was performed in the Department of Surgical Oncology at Institut Curie in Paris, revealing an ES. Conclusion: Mandibular ES can mimic dental infections when swelling is the main clinical manifestation, which can lead to a delayed diagnosis. A correlation between clinical, radiological, histopathological, and immunohistochemical with cytogenetics is needed to confirm the diagnosis. Moreover, smaller tumors have better survival. Dentists must therefore be aware of the clinical signs of ES in order to quickly refer patients to a specialized department.