Gene transfer to create tumour epitope‐specific cytolytic T cells for adoptive immunotherapy of cancer remains an area of active inquiry. When the Mart‐1_27–35‐specific DMF5 T‐cell receptor (TCR) is transferred into peripheral human CD4⁺ T cells, the reprogrammed cells exhibit a T helper type 1 (Th1) phenotype with significant multifactorial effector capabilities. The T‐bet transcription factor plays an important role in determination of the Th1 differentiation pathway. To gain a deeper understanding of how T‐bet controls the outcome of human T‐cell reprogramming by gene transfer, we developed a system for examining the effects of short hairpin RNA‐mediated T‐bet gene knockdown in sorted cell populations uniformly expressing the knockdown construct. In this system, using activated peripheral human CD4⁺ CD25⁻ and CD8⁺ T cells, T‐bet knockdown led to attenuation of the interferon‐γ response to both antigen‐specific and non‐specific TCR stimulation. The interleukin‐2 (IL‐2) antigen‐specific response was not attenuated by T‐bet knockdown. Also, in TCR‐reprogrammed CD8⁺ cells, the cytolytic effector response was attenuated by T‐bet knockdown. T‐bet knockdown did not cause redirection into a Th2 differentiation pathway, and no increased IL‐4, IL‐10, or IL‐17 response was detected in this system. These results indicate that T‐bet expression is required for maintenance of the CD4⁺ CD25⁻ and CD8⁺ effector phenotypes in TCR‐reprogrammed human T cells. They also suggest that the activation protocol necessary for transduction with retrovectors and lentivectors may commit the reprogrammed cells to the Th1 phenotype, which cannot be altered by T‐bet knockdown but that there is, nevertheless, a continuous requirement of T‐bet expression for interferon‐γ gene activation.