CD4+ T cells are critical for host defense but also drive immune-mediated diseases. The classical view of Th1 and Th2 subtypes of CD4+ T cells was recently revised by the identification of the Th17 lineage that produces IL-17 and are critical in autoimmune disease pathogenesis. Mechanisms controlling the differentiation of Th17 cells have been described, but few feasible targets for therapeutically reducing Th17 cells are known. The generation of Th17 cells requires IL-6 and activation of STAT3. During polarization of CD4+ T cells to Th17 cells, we found that inhibition of glycogen synthase kinase-3 (GSK3) blocked IL-6 production, STAT3 activation, and polarization to Th17 cells. Polarization of CD4+ T cells to Th17 cells increased by 10-fold GSK3β protein levels in Th17 cells, while GSK3β was unaltered in Treg cells. Diminishing GSK3 activity pharmacologically or molecularly blocked Th17 cell production, and increasing GSK3 activity promoted polarization to Th17 cells. In vivo inhibition of GSK3 in mice depleted constitutive Th17 cells in intestinal mucosa, blocked Th17 cell generation in the lung following Francisella tularensis infection, and inhibited the increase in spinal cord Th17 cells and disease symptoms in the experimental autoimmune encephalomyelitis (EAE) mouse MS model. These findings identify GSK3 as a critical mediator of Th17 cell production, and indicate that GSK3 inhibitors provide a potential therapeutic intervention to control Th17-mediated diseases.