Suppression by T regulatory cells (Treg cells) is a major mechanism by which the immune system controls responses to self and nonharmful foreign proteins. Although there are many different types of Treg cells, the best characterized are those that constitutively express cell-surface IL-2Rα (CD25). We investigated whether altered T-cell–receptor (TCR)–mediated signaling in pure populations of ex vivo human CD4⁺CD25⁺ Treg cells might underlie their unique phenotype, including hyporesponsiveness to TCR–mediated activation and lack of cytokine production. CD4⁺CD25⁺ Treg cells displayed a consistent defect in phosphorylation of AKT at serine 473 and reduced phosphorylation of the AKT substrates FOXO and S6. Restoration of AKT activity via lentiviral-mediated expression of an inducibly active form of the kinase revealed that reduced activity of this pathway was necessary for the suppressive function of CD4⁺CD25⁺ Treg cells. These data represent the first demonstration of a causal association between altered signaling and the function of CD4⁺CD25⁺ Treg cells. Moreover, we have created the first system allowing inducible abrogation of suppression through manipulation of the suppressor cells. This system will be a powerful tool to further study the mechanism(s) of suppression by CD4⁺CD25⁺ Treg cells.