Ulcerative colitis (UC) is known to be associated with an increased risk of colorectal cancer. However, the occurrence of non-epithelial malignancies is uncommon. An elevated lesion in the descending colon was found in a 51-year-old woman with a 30-year history of UC. Despite tumor progression, repeated biopsies showed no cancerous findings. Because the lesion was highly suspected to be a malignant tumor, a partial colectomy was performed. The pathological diagnosis was leiomyosarcoma. Leiomyosarcoma of the gastrointestinal tract is rare, and this is only the third known case reported in patients with UC.