IL-37, a new member of the IL-1 cytokine family, is a natural inhibitor of innate immunity associated with autoimmune diseases. This study was undertaken to evaluate whether IL-37 has antiarthritic effects in patients with rheumatoid arthritis (RA) and in mice with collagen-induced arthritis (CIA). In this study, we analyzed the expression of IL-37 in PBMCs, serum, and lymphocytes from RA patients as well as CD4+ T cells polarized under Th1/Th2/Th17 conditions. The role of IL-37 was assessed by investigating the effects of recombinant human (rh) IL-37 and an adenovirus encoding human IL-37 (Ad–IL-37) on Th17 cells and Th17-related cytokines in RA patients and CIA mice. We found that active RA patients showed higher IL-37 levels compared with patients with inactive RA and healthy controls. Upregulated IL-37 expression also was found in CD3+ T cells and CD4+ T cells from RA patients and in Th1/Th17-differentiation conditions. rhIL-37 markedly decreased IL-17 expression and Th17 cell frequency in PBMCs and CD4+ T cells from RA patients. Furthermore, IL-37 exerted a more suppressive effect on Th17 cell proliferation, whereas it had little or no effect on Th17 cell differentiation. IL-17 and IL-17–driving cytokine production were significantly reduced in synovium and joint cells from CIA mice receiving injections of Ad–IL-37. Our findings indicate that IL-37 plays a potent immunosuppressive role in the pathogenesis of human RA and CIA models via the downregulation of IL-17 and IL-17–triggering cytokine production and the curbing of Th17 cell proliferation.