Leiomyosarcoma of the rectum is a very rare mesenchymal tumor. Because of its rarity, its diagnosis, treatment, and pathology often present challenges to the clinician. The characteristics of this tumor, such as its anatomical location, heterogeneous solid features on imaging, and nonspecific lower gastrointestinal tract symptoms, can be confused with those of primary ovarian carcinoma. Here, we report the case of a 52-year-old-woman presenting with a low abdominal mass that was later pathologically confirmed to be a rectal leiomyosarcoma. The findings of preoperative ultrasonography, pelvic magnetic resonance imaging, and abdominopelvic computed tomography were suggestive of a malignant pelvic mass, most likely a primary ovarian carcinoma. The patient underwent explorative laparotomy, and intraoperative frozen examination revealed a sarcoma originating from the gastrointestinal tract. Low anterior resection and supracervical hysterectomy with bilateral salpingo-oophorectomy were performed. The patient's postoperative course was uneventful, and adjuvant chemotherapy is currently being administered.