Extraskeletal Ewing sarcoma is a rare soft tissue tumor primarily affecting pediatric patients. The treatment is currently based on a multidisciplinary approach which allows, in cases of localized disease, good survival rates. We report the case of a 15-year-old female patient with a rapidly growing suspected pelvic mass misdiagnosed following the preliminary radiological exams, which assessed the findings as a mass of ovarian origin. The girl underwent surgery and, thanks to histopathological, immunohistochemical and real-time polymerase chain reaction (RT-PCR) examinations, it was possible to make the right diagnosis and to administer the best treatment in terms of surgery, chemotherapy and radiotherapy, obtaining a long disease-free interval and no recurrence to date.