Regulatory T (Treg) cells are essential for maintenance of immune homeostasis. Here we found that hydrogen sulfide (H2S) was required for Foxp3+ Treg cell differentiation and function, and that H2S deficiency led to systemic autoimmune disease. H2S maintained expression of methylcytosine dioxygenases Tet1 and Tet2 by sulfhydrating nuclear transcription factor Y subunit beta (NFYB) to facilitate its binding to Tet1 and Tet2 promoters. Transforming growth factor-β (TGF-β)-activated Smad3 and interleukin-2 (IL-2)-activated Stat5 facilitated Tet1 and Tet2 binding to Foxp3. Tet1 and Tet2 catalyzed conversion of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) in Foxp3 to establish a Treg cell-specific hypomethylation pattern and stable Foxp3 expression. Consequently, Tet1 and Tet2 deletion led to Foxp3 hypermethylation, impaired Treg cell differentiation and function, and autoimmune disease. Thus, H2S promotes Tet1 and Tet2 expression, which are recruited to Foxp3 by TGF-β and IL-2 signaling to maintain Foxp3 demethylation and Treg cell-associated immune homeostasis.