The extra-embryonic yolk sac (YS) is the first hematopoietic site in the mouse embryo and is thought to generate only primitive erythroid and myelo-erythroid progenitor cells prior to definitive hematopoietic stem cell (HSC) emergence within the embryo on embryonic day (E)10.5. Here, we have demonstrated the existence of T cell-restricted progenitors in the E9.5 YS that directly engraft in recipient immunodeficient mice. T cell progenitors were also produced in vitro from both YS and para-aortic splanchnopleura (P-Sp) hemogenic endothelial cells, and these T cell progenitors repopulated the thymus and differentiated into mature T cell subsets in vivo upon transplantation. Our data confirms that the YS produces T-lineage restricted progenitors that are available to colonize the thymus, and provides new insight into the YS as a definitive hematopoietic site in the mouse embryo