Transforming growth factor beta (TGF-β), a multifunctional cytokine, plays critical roles in immune responses. However, the precise role of TGF-β in colitis and colitis-associated cancer remains poorly defined. Here, it is demonstrated that TGF-β promotes the colonic inflammation and related tumorigenesis in the absence of Smad family member 4 (Smad4). Smad4 loss in intestinal epithelium aggravates colitis and colitis-associated neoplasia induced by dextran sulfate sodium (DSS) and azoxymethane/dextran sulfate sodium (AOM/DSS), leading to over-activated immune responses and increased TGF-β1 levels. In Smad4-deficient organoids, TGF-β1 stimulates spheroid formation and impairs intestinal stem cell proliferation and lineage specification. YAP, whose expression is directly upregulated by TGF-β1 after Smad4 deletion, mediates the effect of TGF-β1 by interacting with Smad2/3. Attenuation of YAP/TAZ prevents TGF-β1-induced spheroid formation in Smad4-/- organoids and alleviates colitis and colitis-associated cancer in Smad4-deficient mice. Collectively, these results highlight an integral role of the TGF-β/Smad4 axis in restraining intestinal inflammation and tumorigenesis and suggest TGF-β or YAP signaling as therapeutic targets for these gastrointestinal diseases intervention.Keywords: Smad family member 4; YAP/TAZ; colitis-associated cancer; inflammatory bowel disease; transforming growth factor beta.