Abstract: Th9 cells, a novel subset of CD4+ T cells, mainly produceIL-9 and have a critical role in allergic disease, autoimmune disease, and tumors. IL-9 can promote inflammation by increasing the number of inflammatory cells and the secretion of inflammatory mediators and thus play an important role in the development of inflammatory bowel disease (IBD). Th9 cells and IL-9 aggravate inflammation of the colon in both experimental models of colitis and in patients with IBD. The blockade of IL-9 is suited to significantly ameliorate the disease activity and severity of IBD.