Patients with inflammatory bowel disease (ulcerative colitis, Crohn’s disease) have decreased production of endogenous aryl hydrocarbon receptor (AhR) ligands in colon tissue. Activation of the AhR is a potential novel therapeutic target for ulcerative colitis and may play an important role in promoting immune homeostasis through increased IL-22 production, downregulation of proinflammatory mediators, and improved epithelial barrier function. Indigo naturalis, a botanical extract containing potent AhR agonists, has been effective for the treatment of moderate-to-severe and treatment-refractory ulcerative colitis in multiple clinical studies.