The transcription factors GATA-3 and ThPOK are required for intrathymic differentiation of CD4⁺ T cells, but their precise functions in this process remain unclear. Here we show that, contrary to previous findings, Gata3 disruption blocked differentiation into the CD4⁺ T cell lineage before commitment to the CD4⁺ lineage and in some contexts permitted the 'redirection' of major histocompatibility complex class II–restricted thymocytes into the CD8⁺ lineage. GATA-3 promoted ThPOK expression and bound to a region of the locus encoding ThPOK established as being critical for ThPOK expression. Finally, ThPOK promoted differentiation into the CD4⁺ lineage in a way dependent on GATA-3 but inhibited differentiation into the CD8⁺ lineage independently of GATA-3. We propose that GATA-3 acts as a specification factor for the CD4⁺ lineage 'upstream' of the ThPOK-controlled CD4⁺ commitment checkpoint.