SUMMARY: Coreceptor CD4 and CD8αβ double-negative (DN) TCRαβ intraepithelial T cells, although numerous, have been greatly overlooked and their contribution to the immune response is not known. Here we used T cell receptor (TCR) sequencing of single cells combined with retrogenic expression of TCRs to study the fate and the major histocompatibility complex (MHC) restriction of DN TCRαβ intraepithelial T cells. The data show that commitment of thymic precursors to the DN TCRαβ lineage is imprinted by their TCR specificity. Moreover, the TCRs they express display a diverse and unusual pattern of MHC restriction that is nonoverlapping with that of CD4 or CD8αβ T cells, indicating that they sense antigens that are not recognized by the conventional T cell subsets. The new insights indicate that DN TCRαβ T cells form a third lineage of TCRαβ T lymphocytes expressing a variable TCR repertoire, which serve nonredundant immune functions. GRAPHICAL ABSTRACT: (Figure is included in full-text article.) HIGHLIGHTS: : How intraepithelial coreceptor-negative TCRαβ T cells develop is unclear. Lambolez and colleagues demonstrate that commitment to this lineage occurs in the thymus and is imprinted by T cell receptor (TCR) specificity and that these TCRs have unique MHC restriction, suggesting a nonredundant role in protective immunity.