Thymoma is often associated with other diseases, including autoimmune diseases and endocrine disorders. Ulcerative colitis is a nonspecific inflammatory disease and is thought to be an immune-mediated disorder; however, coincidence of thymoma and ulcerative colitis is rare. We treated a 51-year-old woman with invasive thymoma and ulcerative colitis. After excision of the invasive thymoma with partial resections of pericardium, left lung, brachiocephalic vein, and left phrenic nerve, the ulcerative colitis was cured.