TET enzymes oxidize 5-methylcytosine to 5-hydroxymethylcytosine and other oxidized methylcytosines in DNA. Here we examine the role of TET proteins in regulatory T (Treg) cells. Tet2/3^fl/flFoxp3^Cre mice lacking Tet2 and Tet3 in Treg cells develop inflammatory disease, and Treg cells from these mice show altered expression of Treg signature genes and upregulation of genes involved in cell cycle, DNA damage and cancer. In littermate mice with severe inflammation, both CD4⁺Foxp3⁺ and CD4⁺Foxp3⁻ cells show strong skewing towards Tfh/Th17 phenotypes. Wild-type Treg cells in mixed bone marrow chimeras and in Tet2/3^fl/flFoxp3^WT/Cre heterozygous female mice are unable to rescue the aberrant properties of Tet2/3^fl/flFoxp3^Cre Treg cells. Treg cells from Tet2/3^fl/flFoxp3^Cre mice tend to lose Foxp3 expression, and transfer of total CD4⁺ T cells isolated from Tet2/3^fl/flFoxp3^Cre mice could elicit inflammatory disease in fully immunocompetent mice. Together, these data indicate that Tet2 and Tet3 are guardians of Treg cell stability and immune homeostasis.