Undifferentiated embryonal sarcoma of the liver (UESL) is a rare and highly aggressive malignant mesenchymal neoplasm that occurs mainly in school-aged children. Preoperative diagnosis is challenging because clinical symptoms are nonspecific and imaging findings can vary substantially. Despite these diagnostic difficulties, survival has improved markedly with modern multimodal treatment. A previously healthy 10-year-old girl presented with acute right upper quadrant abdominal pain. Laboratory findings were largely unremarkable, and the alpha-fetoprotein level was within the normal range. Contrast-enhanced abdominal computed tomography and magnetic resonance imaging showed a large heterogeneous cystic mass in the right hepatic lobe, with internal hemorrhage, hemorrhage-fluid levels, and enhancing solid components. Right hepatectomy was performed, and complete tumor resection was achieved. Histopathological evaluation revealed a high-grade malignant mesenchymal tumor with pathological and immunohistochemical features consistent with UESL. The patient subsequently received adjuvant chemotherapy. At 21 months after surgery, she remained disease-free, with no radiological or clinical evidence of recurrence. This case highlights the diagnostic value of characteristic imaging findings and the therapeutic importance of complete resection followed by adjuvant chemotherapy. Early recognition and coordinated multidisciplinary management are essential for improving prognosis in children with this rare hepatic malignancy.