Memory stem T cells (T_SCM) have been proposed as key determinants of immunologic memory. However, their exact contribution to a mounting immune response, as well as the mechanisms and timing of their in vivo generation, are poorly understood. We longitudinally tracked T_SCM dynamics in patients undergoing haploidentical hematopoietic stem cell transplantation (HSCT), thereby providing novel hints on the contribution of this subset to posttransplant immune reconstitution in humans. We found that donor-derived T_SCM are highly enriched early after HSCT. We showed at the antigen-specific and clonal level that T_SCM lymphocytes can differentiate directly from naive precursors infused within the graft and that the extent of T_SCM generation might correlate with interleukin 7 serum levels. In vivo fate mapping through T-cell receptor sequencing allowed defining the in vivo differentiation landscapes of human naive T cells, supporting the notion that progenies of single naive cells embrace disparate fates in vivo and highlighting T_SCM as relevant novel players in the diversification of immunological memory after allogeneic HSCT.