Tcf1 and Lef1 have versatile functions in regulating T cell development and differentiation, but intrinsic requirements for these factors in regulatory T (T reg) cells remain to be unequivocally defined. Specific ablation of Tcf1 and Lef1 in T reg cells resulted in spontaneous multi-organ autoimmunity that became more evident with age. Tcf1/Lef1-deficient T regs showed reduced protection against experimentally induced colitis, indicative of diminished immuno-suppressive capacity. Transcriptomic analysis revealed that Tcf1 and Lef1 were responsible for positive regulation of a subset of T reg–overrepresented signature genes such as Ikzf4 and Izumo1r. Unexpectedly, Tcf1 and Lef1 were necessary for restraining expression of cytotoxic CD8+ effector T cell–associated genes in T reg cells, including Prdm1 and Ifng. Tcf1 ChIP-seq revealed substantial overlap between Tcf1 and Foxp3 binding peaks in the T reg cell genome, with Tcf1-Foxp3 cooccupancy observed at key T reg signature and cytotoxic effector genes. Our data collectively indicate that Tcf1 and Lef1 are critical for sustaining T reg suppressive functions and preventing loss of self-tolerance.