Allogeneic hematopoietic stem cell transplantation (HSCT) is a curative treatment for multiple disorders, but deficiency and dysregulation of T cells limit its utility. Here we report a biomaterial-based scaffold that mimics features of T cell lymphopoiesis in the bone marrow. The bone marrow cryogel (BMC) releases bone morphogenetic protein-2 to recruit stromal cells and presents the Notch ligand Delta-like ligand-4 to facilitate T cell lineage specification of mouse and human hematopoietic progenitor cells. BMCs subcutaneously injected in mice at the time of HSCT enhanced T cell progenitor seeding of the thymus, T cell neogenesis and diversification of the T cell receptor repertoire. Peripheral T cell reconstitution increased ~6-fold in mouse HSCT and ~2-fold in human xenogeneic HSCT. Furthermore, BMCs promoted donor CD4⁺ regulatory T cell generation and improved survival after allogeneic HSCT. In comparison to adoptive transfer of T cell progenitors, BMCs increased donor chimerism, T cell generation and antigen-specific T cell responses to vaccination. BMCs may provide an off-the-shelf approach for enhancing T cell regeneration and mitigating graft-versus-host disease in HSCT.