We report a case of ulcerative colitis (UC) that occurred during the course of rheumatoid arthritis (RA). A 29-year-old woman with a 25-year history of RA was hospitalized for high fever, abdominal pain and hematochezia. Colonoscopy revealed erosive and reddish mucosa from the distal transverse colon to rectum. Histology revealed cryptitis (mainly caused by neutrophils), mild crypt abscess and goblet cell depletion. She was diagnosed with left-sided UC and treated with mesalamine enema. The abdominal symptoms and colonoscopic findings were greatly ameliorated. We conclude that the mesalamine enema was effective in this case of UC occurring during the course of RA.