Primary choriocarcinoma of the sigmoid colon is an extremely rare malignancy, with only eight cases previously reported worldwide. The prognosis for this malignancy is usually poor, and there is no standard treatment. A 44-year-old woman presented with nausea and vaginal bleeding. Because she had a uterine mass and a high serum concentration of human ß-chorionic gonadotropin (428,864mIU/ml), she was incorrectly diagnosed with a uterine choriocarcinoma. Immunohistochemical assays resulted in a diagnosis of primary adenocarcinoma with choriocarcinomatous differentiation in the sigmoid colon, along with liver and lung metastases and secondary degeneration of submucosal myoma. She underwent total abdominal hysterectomy plus anterior resection of the colon. Because of her poor general condition, she received only a single cycle of cisplatin chemotherapy. She died of multiorgan failure due to multiple metastases 2 months after the initial diagnosis. Early diagnosis of primary choriocarcinoma of the sigmoid colon requires a multidisciplinary approach and is associated with patient prognosis.