A woman in her 60s visited the gynecology department of our hospital with a chief complaint of lower abdominal pain and bloating. Contrast-enhanced CT and MRI of the abdomen revealed an irregularly shaped 10 cm mass in contact with the uterus. Presence of a poorly contrast-enhanced area within the mass led to the diagnosis of uterine sarcoma, and surgical intervention was undertaken. A combined resection of the uterus and bilateral ovaries was performed; however, the tumor was found to be contiguous with the sigmoid mesentery rather than the uterus. Therefore, the sigmoid colon, including the tumor, was resected. No evidence of metastases or peritoneal dissemination was observed. Histopathological examination revealed spindle-shaped cells with strong nuclear atypia. Leiomyosarcoma was diagnosed based on h-caldesmon (+), α-SMA(+), desmin(+), and c-kit(-). One year after surgery, the patient is alive without recurrence.