Ulcerative colitis, or simply 'colitis', is also an inflammatory condition, but is confined to the inner lining of the large intestine and rectum. It is almost always present in the rectum and tends to be continuous in appearance involving either the lower part only, or extending further along the entire length of the large intestine.

When limited to the rectum, the correct term is 'ulcerative proctitis'. In a few individuals it is not possible to distinguish whether they have Crohn's disease or ulcerative colitis. The condition is then referred to as indeterminate colitis.