IL-23, an IL-12-related cytokine, induces an IL-17-secreting T-helper phenotype that is involved in autoimmune diseases and host defense against certain pathogens. Although the transcription factors required for development of IL-23-stimulated cells are unknown, we show that T-bet is a critical negative regulator of the IL-23-primed T-cell phenotype, which we term Th1β. Th1 or Th1β Tbx21^-/- cultures secrete higher than WT levels of IL-17 in response to T-cell receptor (TCR) or IL-23 + IL-18 stimulation. Ectopic T-bet expression in Th1β cells promotes IFN-γ secretion but decreases IL-17 production. Although antigen-receptor stimulation of Th1β cells stimulates IL-17 production, it also induces the IFN-γ-independent expression of T-bet and progression to a Th1 cytokine secretion pattern. T-bet is required for the progression to the Th1 phenotype, because Tbx21^-/- Th1β cultures maintain the IL-17-secreting phenotype after 2 weeks of culture. Addition of IFN-γ to Tbx21^-/- Th1β cultures cannot recover the progression to the Th1 phenotype, suggesting T-bet, rather than IFN-γ, mediates Th1β to Th1 progression. The transient nature of the Th1β phenotype suggests that these cells are a component of type I immunity and that T-bet expression is a critical determinant of Th1 versus Th1β cell fate.