Sarcomas of the broad ligament are very uncommon. To our knowledge, there are no cases published of undifferentiated round cell sarcoma of the broad ligament. Round cell sarcomas are a rare and very aggressive variant, which due to their sensitivity to chemotherapy, have an acceptable prognosis. We report the case of a 27-year-old woman who presented with a pelvic mass with a 7-cm diameter placed on the right broad ligament. After surgery, she was diagnosed with undifferentiated round cell sarcoma of the broad ligament. The patient received adjuvant chemotherapy and radiotherapy, and after 12 years of follow-up, she still remains asymptomatic. Proper differential diagnoses as well as an appropriate adjuvant therapy after surgical treatment seem to be essential to obtain good oncological outcomes in this rare entity.