In naïve animals, γδ T cells are innate sources of IL-17, a potent proinflammatory cytokine mediating bacterial clearance as well as autoimmunity. However, mechanisms underlying the generation of these cells in vivo remain unclear. Here we show that TGFβ1 plays a key role in the generation of IL-17+ γδ T cells, and that it mainly occurs in the thymus particularly during the postnatal period. Interestingly, IL-17+ γδ TCR+ thymocytes were mainly CD44highCD25low cells, which seem to derive from DN4 γδ TCR+ cells that acquired CD44 and IL-17 expression. Our findings identify a novel developmental pathway during which IL-17-competent γδ T cells arise in the thymus by a TGFβ1-dependent mechanism.