BACKGROUND: GPR171 suppresses T cell immune responses involved in antitumour immunity, while its role in inflammatory bowel disease (IBD) pathogenesis remains unclear. OBJECTIVE: We aimed to investigate the role of GPR171 in modulating CD4(+) T cell effector functions in IBD and evaluate its therapeutic potential. DESIGN: We analysed GPR171 expression in colon biopsies and peripheral blood samples from patients with IBD and assessed the impact of GPR171 on CD4(+) T cell differentiation through administration of its endogenous ligand (BigLEN). We further determined the role of GPR171 in dextran sulfate sodium (DSS)-induced colitis and CD45RB(high)CD4(+) T-cell transfer colitis model and deciphered the underlying mechanisms using RNA sequencing (RNA-seq) and lipidomics. We developed a novel BigLEN-based Fc fusion protein (BigLEN-Fc) and evaluated its potential in preventing and treating colitis. RESULTS: GPR171 was markedly increased in inflamed mucosa and CD4(+) T cells of patients with IBD compared with controls. BigLEN-triggered GPR171 activation inhibited Th17 cell differentiation in vitro. GPR171 deficiency exacerbated DSS- and CD45RB(high)CD4(+) T cell-induced colitis in mice, characterised by increased Th17 cell responses in intestinal mucosa. Mechanistically, GPR171 deficiency promoted Th17 cell differentiation and altered lipidome profile in Th17 cells via the cAMP-pCREB-FABP5 axis. Blockage of FABP5 reduced Th17 cell differentiation in vitro and ameliorated DSS-induced colitis in Gpr171 (-/-) mice. Furthermore, BigLEN-mutFc administration potently mitigated colitis in mice. CONCLUSIONS: GPR171 deficiency promotes Th17 cell differentiation and causes lipid metabolism perturbation, contributing to intestinal inflammation in a FABP5-dependent manner. Target therapy (eg, BigLEN-Fc) represents a novel therapeutic approach for IBD treatment.