Interferon regulatory factor 4 (IRF4) is a transcription factor that is expressed in hematopoietic cells and plays pivotal roles in the immune response. Originally described as a lymphocyte‐specific nuclear factor, IRF4 promotes differentiation of naïve CD4⁺ T cells into T helper 2 (Th2), Th9, Th17, or T follicular helper (Tfh) cells and is required for the function of effector regulatory T (eTreg) cells. Moreover, IRF4 is essential for the sustained differentiation of cytotoxic effector CD8⁺ T cells, for CD8⁺ T‐cell memory formation, and for differentiation of naïve CD8⁺ T cells into IL‐9‐producing (Tc9) and IL‐17‐producing (Tc17) CD8⁺ T‐cell subsets. In this review, we focus on recent findings on the role of IRF4 during the development of CD4⁺ and CD8⁺ T‐cell subsets and the impact of IRF4 on T‐cell‐mediated immune responses in vivo.