The authors describe a case of uterine leiomyosarcoma, in which clinical, radiological, histopathological, and genetic findings suggested malignant transformation of uterine leiomyoma as the cause of the disease. A 37-year-old woman receiving follow-up care for a uterine leiomyoma detected six years ago presented with lower abdominal pain. Sequential magnetic resonance imaging scans indicated the development of uterine leiomyosarcoma in the same location as the preexisting uterine leiomyoma. Histopathological examination of the excised tumor revealed uterine leiomyosarcoma with a component of uterine leiomyoma. Concordant mediator subcomplex 12 gene mutation status was observed in both components. This report supports the possibility of malignant transformation of uterine leiomyoma to uterine leiomyosarcoma.