Novel therapies for chronic graft-versus-host disease (cGVHD) are needed. Aberrant B-cell activation has been demonstrated in mice and humans with cGVHD. Having previously found that humancGVHDBcells are activated and primed for survival, we sought to further evaluate the role of the spleen tyrosine kinase (Syk) in cGVHD in multiple murine models and human peripheral blood cells. In a murine model of multiorgan system, nonsclerodermatous disease with bronchiolitis obliterans where cGVHD is dependent on antibody and germinal center (GC) B cells, we found that activation of Syk was necessary in donor B cells, but not T cells, for disease progression. Bonemarrow-specific Syk deletion in vivo was effective in treating established cGVHD, as was a small-molecule inhibitor of Syk, fostamatinib, which normalized GC formation and decreased activated CD80/861 dendritic cells. Inmultiple distinctmodels of sclerodermatouscGVHD, clinical and pathological diseasemanifestationswere not eliminated when mice were therapeutically treated with fostamatinib, though both clinical and immunologic effects could be observed in one of these sclerodermamodels. We furtherdemonstratedthatSykinhibitionwaseffectiveat inducingapoptosisofhumancGVHDBcells.Together, these data demonstrate a therapeutic potential of targeting B-cell Syk signaling in cGVHD.