BACKGROUND & AIMS: Retinoblastoma-binding protein 9 (RBBP9) was initially reported as cell cycle regulator via RB/E2F. Accumulating evidence has revealed the importance of RBBP9 in physiological and pathological states including inflammatory disease. However, the functional role of RBBP9 in ulcerative colitis (UC) and colitis-associated cancer (CAC) remains elusive. METHODS: Human samples of UC and CAC were examined by immunohistochemical and bioinformatics analyses. We established dextran sodium sulfate (DSS)-induced colitis, azoxymethane (AOM)/DSS-induced CAC model, and Apc RESULTS: The expression of RBBP9 was reduced in human UC and CAC samples. The loss of RBBP9 enhanced the activation of interferon (IFN)/JAK/STAT1 signaling, resulting in susceptibility to DSS-induced colitis and AOM/DSS-induced CAC tumors by increasing epithelial cell apoptosis and immune activation. An in vitro kinase assay revealed that RBBP9 directly regulated JAK/STAT1 signaling by suppressing STAT1 phosphorylation. A positive feedback loop involving epithelial cell apoptosis, commensal microbiome invasion, and activation of submucosal immune activity was identified in Rbbp9 CONCLUSIONS: RBBP9 suppresses the intestinal inflammation by negatively regulating JAK/STAT1 signaling pathway.