We report a case of undifferentiated sarcoma originating from the mitral valve in a 3-year-old boy who presented with embolic symptoms of the lower extremity. The tumor was demonstrated by echocardiography. The mass was removed under a mild hypothermic cardiopulmonary bypass with cold blood cardioplegia. Grossly, the mass was composed of multiple small nodules, and the histopathologic findings were compatible with an undifferentiated sarcoma. Postoperative chemotherapy was added. The patient remains without evidence of recurrence 18 months after surgery.