Background: Uterine leiomyosarcoma (uLMS) is a rare and aggressive malignancy, accounting for approximately 1% of all gynecological malignancies and 1–2% of uterine tumors. It is frequently diagnosed at an advanced stage, contributing to its poor prognosis. Preoperative differentiation from benign uterine leiomyomas remains challenging, as no imaging modality has proven suffciently reliable to exclude malignancy. Case Presentation: We report a case of a 69-year-old grand multiparous postmenopausal woman who presented with postmenopausal bleeding and pelvic pain. She had previously undergone exploratory laparotomy six months earlier, during which a retroperitoneal mass causing compression of the iliac vessels was biopsied. Histological analysis revealed a spindle-cell mesenchymal proliferation with immunohistochemical features consistent with smooth muscle differentiation. Pelvic MRI demonstrated a large left-lateralized abdominopelvic mass measuring 17×15×15 cm. She subsequently underwent total hysterectomy with bilateral salpingo-oophorectomy. Final pathological examination confirmed a uterine leiomyosarcoma measuring 19 cm, with tumor necrosis, vascular emboli, and full-thickness myometrial infiltration. Ki-67 proliferation index was 40%. Conclusion: This case highlights the diagnostic and therapeutic challenges posed by uterine leiomyosarcoma presenting as a large retroperitoneal mass. Total hysterectomy without ovarian conservation remains the cornerstone of treatment. Multidisciplinary collaboration and long-term oncological follow-up are essential in the management of this rare and aggressive neoplasm.