Early thymocytes possess multilineage potential, which is progressively restricted as cells transit through the double-negative stages of T-cell development. DN1 cells retain the ability to become natural killer cells, dendritic cells, B cells, and myeloid cells as well as T cells, but these options are lost by the DN3 stage. The Notch1 signaling pathway is indispensable for initiation of the T-cell lineage and inhibitory for the B-cell lineage, but the regulatory mechanisms by which the T-cell fate is locked in are largely undefined. Previously, we discovered that the E-protein transcription factor HEBAlt promoted T-cell specification. Here, we report that HEB^−/− T-cell precursors have compromised Notch1 function and lose T-cell potential. Moreover, reconstituting HEB^−/− precursors with Notch1 activity enforced fidelity to the T-cell fate. However, instead of becoming B cells, HEB^−/− DN3 cells adopted a DN1-like phenotype and could be induced to differentiate into thymic NK cells. HEB^−/− DN1-like cells retained GATA3 and Id2 expression but had lower levels of the Bcl11b gene, a Notch target gene. Therefore, our studies have revealed a new set of interactions between HEB, Notch1, and GATA3 that regulate the T-cell fate choice in developing thymocytes.