Follicular helper T cells (T_FH cells) are specialized effector CD4⁺ T cells that help B cells develop germinal centers (GCs) and memory. However, the transcription factors that regulate the differentiation of T_FH cells remain incompletely understood. Here we report that selective loss of Lef1 or Tcf7 (which encode the transcription factor LEF-1 or TCF-1, respectively) resulted in T_FH cell defects, while deletion of both Lef1 and Tcf7 severely impaired the differentiation of T_FH cells and the formation of GCs. Forced expression of LEF-1 enhanced T_FH differentiation. LEF-1 and TCF-1 coordinated such differentiation by two general mechanisms. First, they established the responsiveness of naive CD4⁺ T cells to T_FH cell signals. Second, they promoted early T_FH differentiation via the multipronged approach of sustaining expression of the cytokine receptors IL-6Rα and gp130, enhancing expression of the costimulatory receptor ICOS and promoting expression of the transcriptional repressor Bcl6.