BACKGROUND Mixed endometrial stromal and smooth muscle tumors (MESSMT) of the uterus are rare disease entities. The histogenesis is unclear, but its clinical manifestations are similar to those of other mesenchymal tumors. This unique uterine tumor was originally reported as having ultrastructural characteristics of both endometrial stromal and smooth muscle cells. Subsequently, MESSMT was defined as having at least 30% of each component present. Here we present an MESSMT with unusual features in a nulliparous woman and describe its morphological and immunohistochemical characteristics. CASE REPORT A 35-year-old nulliparous woman presented with menorrhagia for 2 months. Transvaginal ultrasonography and magnetic resonance imaging revealed an enlarged uterus with a 6.0×5.5×4.5 cm mass and cystic degeneration. The patient underwent abdominal mass excision. Microscopically, the tumor consisted of 3 distinct components. The outermost area consisted of smooth muscle cells. Well-differentiated endometrial stromal cells that were centrally located showed irregular borders and were merged with smooth muscle cells. In addition, the endometrial stromal component showed focal sex-cord-like differentiation. Morphological and immunohistochemical evaluations were performed, and a MESSMT with focal sex-cord-like differentiation was diagnosed. The patient's postoperative course was uneventful for 29 months. CONCLUSIONS The diagnosis of MESSMT is challenging due to its many overlapping features with other mesenchymal uterine tumors. Although this rare tumor was histologically and clinically consistent with low-grade endometrial stromal sarcoma, it can cause recurrence and metastasis. Therefore, regular follow-up with radiologic examination is essential for the timely detection of local recurrence and distant metastasis.