Malignant transformation is an infrequent complication of endometriosis. The ovary is the primary site in 76% of cases, and extragonadal sites are identified in 24%. Endometrioid carcinoma is the most common histologic type; sarcoma is very rare. We report a case of low-grade endometrial stromal sarcoma of the rectosigmoid colon presenting with epigastric pain due to portal vein thrombosis. This tumor arose from extragonadal endometriosis in a 61-year-old woman and was treated by surgical resection. The main differential diagnosis of this unusual colonic neoplasm includes primary mesenchymal tumors, such as gastrointestinal stromal tumors.