A 24-year-old woman presented with continuous vaginal bleeding and anemia. Pelvic sonography demonstrated an enlarged uterus measuring 8.9 x 4.9 x 4.7 cm and a mass within the endometrial cavity measuring 3.0 x 1.8 cm. The mass appeared to be polypoid and echogenic, contained a small cyst, and was suggestive of a polyp. The patient underwent total vaginal hysterectomy. Pathologic examination revealed a uterine sex cord-stromal tumor. To our knowledge, the sonographic features of uterine sex cord-stromal tumor have not been previously reported. The sonographic appearance mimicked that of a polyp.