Undifferentiated embryonal sarcoma (UES) of the liver was first identified as an independent clinicopathologic type of sarcoma in 1978. It is an uncommon hepatic tumor, of mesenchymal origin, usually observed in children, and cases in adults are rare: to our best knowledge, reports of only 51 cases have been published in the past 50 years. We present a case of UES of the liver in a previously healthy 22 year-old woman, admitted to our hospital due to a palpable mass in the right upper abdomen. On admission, laboratory studies showed mildly elevated aspartate aminotransferase, alkaline phosphatase, and gamma-GPT. Hepatitis and tumor markers were negative. Ultrasonography (US), computed tomography (CT), and magnetic resonance imaging (MRI) showed a large mass involving the right lobe and the medial segment of the liver, with compression of the bile duct. Right trisectionectomy with bile duct resection and reconstruction was performed. Microscopically, the tumor was composed of pleomorphic spindle cells in a myxoid stroma with focal staining of S-100 by immunohistochemistry. The histologic diagnosis was UES. Adjuvant therapy with vincristine, actinomycin-D, and cyclophosphamide was performed, and at 14 months of follow-up, the patient is alive without any evidence of recurrence. The clinical and histopathological features, as well as the therapeutic choices for adult UES, are described for this patient and in the literature of the past 50 years.