Acute Graft-versus-host disease (GVHD) is a major immunological complication after allogeneic hematopoietic cell transplantation and a better understanding of the molecular regulation of the disease could help to develop novel targeted therapies. Here we found that a G/C polymorphism within the human microRNA-146a (miR-146a) gene of transplant recipients, which causes reduced miR-146a levels, was strongly associated with the risk of developing severe acute GVHD (n=289). In mice, deficiency of miR-146a in the hematopoietic system or transfer of recipient-type miR-146a−/− dendritic cells (DCs) enhanced GVHD, while miR-146a mimic-transfected DCs ameliorated disease. Mechanistically, lack of miR-146a enhanced JAK2–STAT1 pathway activity, which led to higher expression of class II-transactivator (CIITA) and consecutively increased MHCII-levels on DCs. Inhibition of JAK1/2 or CIITA knockdown in DCs prevented miR-146a−/− DC-induced GVHD exacerbation. Consistent with our findings in mice, patients with the miR-146a polymorphism rs2910164 in hematopoietic cells displayed higher MHCII levels on monocytes, which could be targeted by JAK1/2 inhibition. Our findings indicate that the miR-146a polymorphism rs2910164 identifies patients at high risk for GVHD before allo-HCT. Functionally we show that miR-146a acts as a central regulator of recipient-type DC activation during GVHD by dampening the pro-inflammatory JAK–STAT/CIITA/MHCII axis, which provides a scientific rationale for early JAK1/2 inhibition in selected patients.