Dysregulated CD4⁺ T cell responses and alterations in T regulatory cells (T_reg cells) play a critical role in autoimmune diseases, including inflammatory bowel disease (IBD). The current study demonstrates that removal of Bcl11b at the double-positive stage of T cell development or only in T_reg cells causes IBD because of proinflammatory cytokine-producing CD4⁺ T cells infiltrating the colon. Provision of WT T_reg cells prevented IBD, demonstrating that alterations in T_reg cells are responsible for the disease. Furthermore, Bcl11b-deficient T_reg cells had reduced suppressor activity with altered gene expression profiles, including reduced expression of the genes encoding Foxp3 and IL-10, and up-regulation of genes encoding proinflammatory cytokines. Additionally, the absence of Bcl11b altered the induction of Foxp3 expression and reduced the generation of induced T_reg cells (iT_reg cells) after Tgf-β treatment of conventional CD4⁺ T cells. Bcl11b bound to Foxp3 and IL-10 promoters, as well as to critical conserved noncoding sequences within the Foxp3 and IL-10 loci, and mutating the Bcl11b binding site in the Foxp3 promoter reduced expression of a luciferase reporter gene. These experiments demonstrate that Bcl11b is indispensable for T_reg suppressor function and for maintenance of optimal Foxp3 and IL-10 gene expression, as well as for the induction of Foxp3 expression in conventional CD4⁺ T cells in response to Tgf-β and generation of iT_reg cells.