Receptors of the Notch family direct the differentiation of helper T cell subsets, but their influence on regulatory T cell (T reg cell) responses is obscure. We found here that lineage-specific deletion of components of the Notch pathway enhanced T reg cell–mediated suppression of type 1 helper T cell (T H 1 cell) responses and protected against their T H 1 skewing and apoptosis. In contrast, expression in T reg cells of a gain-of-function transgene encoding the Notch1 intracellular domain resulted in lymphoproliferation, exacerbated T H 1 responses and autoimmunity. Cell-intrinsic canonical Notch signaling impaired T reg cell fitness and promoted the acquisition by T reg cells of a T H 1 cell–like phenotype, whereas non-canonical Notch signaling dependent on the adaptor Rictor activated the kinase AKT–transcription factor Foxo1 axis and impaired the epigenetic stability of Foxp3. Our findings establish a critical role for Notch signaling in controlling peripheral T reg cell function.