BACKGROUND & AIMS: Death receptor 3 (DR3) and its ligand, tumor necrosis factor-like ligand 1A (TL1A), regulate the balance between effector and regulatory T cells in inflammatory bowel disease (IBD). Although interleukin 9 (IL9)-secreting T helper 9 (Th9) cells are linked to ulcerative colitis, their role in Crohn's disease (CD) is unclear. We investigated the role of DR3 signaling in Th9 cell differentiation in mouse models of CD-like ileitis and colitis. METHODS: Polarized Th9 cells with functional DR3 and DR3-deficient Th9 cells from SAMP wild-type (Th9(WT)) and DR3(-/-)xSAMP knockout (Th9(KO)) mice, respectively, were characterized and adoptively transferred into Rag2(-/-) and SAMPxRag2(-/-) recipients. Expression of Th9-associated molecules from experimental mice and IBD patients/controls was compared. RESULTS: Th9(WT) possess a proinflammatory profile compared with Th9(KO) cells; conversely, ablation of DR3 signaling generates anti-inflammatory responses, as reflected by increased IL10-producing cells in DR3(-/-)xSAMP mice. RNA sequencing and phosphoproteomic analyses show that inflammatory pathways are robustly activated in Th9(WT) compared with Th9(KO) cells, whereas Th9 cells are detected in SAMP mice in vivo, and Th9-related genes display the same expression patterns in both experimental ileitis and IBD patients. Finally, in the T-cell adoptive transfer model, Th9(KO) cells are less colitogenic than Th9(WT), whereas IL9 blockade diminishes the severity of intestinal inflammation, indicating a crucial role of functional DR3 receptor in the pathogenicity of Th9 cells. CONCLUSIONS: We demonstrate that the functional DR3 receptor is essential for Th9 cell pathogenicity, revealing a new mechanism by which TL1A/DR3 signaling drives experimental CD-like ileitis. The TL1A/DR3/Th9 proinflammatory pathway may offer a novel therapeutic target for patients with CD.