Targeting T-cell is a strategy to control allogeneic response disorders, such as acute graft-versus-host disease (GVHD) which is an important cause of therapy-failure after allogeneic hematopoietic cell transplants. Free fatty acid receptor-4 (FFAR4) is a regulator of obesity but its role in T-cell and allogeneic reactions is unknown. Here, we found knockout of Ffar4 in donor T-cells in a mouse allograft model increased acute GVHD whereas the natural FFAR4 ligands and the synthetic FFAR4 agonists decreased it. FFAR4 agonist-mediated anti-acute GVHD effects depended on FFAR4-expression in donor T-cells. The FFAR4 agonist CpdA suppressed donor T-cell-mediated alloreaction by activating an aryl hydrocarbon receptor (AhR) pathway. CpdA recruited β-Arrestin2 to FFAR4 which facilitated nuclear translocation of AhR and upregulation of IL-22. The CpdA-mediated anti-acute GVHD effect was absent in mice receiving Ahr-knockout or Il22-knockout T-cells. Recipient-expressing Ffar4 was also important for the anti-acute GVHD effect of CpdA which inhibited activation of antigen presenting cells. Importantly, CpdA decreased acute GVHD in obese mice, an effect also depended on Ffar4-expression in donor T-cells and recipients. Our study shows the immunoregulatory effect of FFAR4 in T-cell, and targeting FFAR4 might be a relative option for controlling allogeneic reactions in obese patients.