Regulatory T (T_reg) cells constitute a dynamic population that is critical in autoimmunity. T_reg cell therapies for autoimmune diseases are mainly focused on enhancing their suppressive activities. However, recent studies demonstrated that certain inflammatory conditions induce T_reg cell instability with diminished FoxP3 expression and convert them into pathogenic effector cells. Therefore, the identification of novel targets crucial to both T_reg cell function and plasticity is of vital importance to the development of therapeutic approaches in autoimmunity. In this study, we found that conditional Pp6 knockout (cKO) in T_reg cells led to spontaneous autoinflammation, immune cell activation, and diminished levels of FoxP3 in CD4⁺ T cells in mice. Loss of Pp6 in T_reg cells exacerbated two classical mouse models of T_reg related autoinflammation. Mechanistically, Pp6 deficiency increased CpG motif methylation of the FoxP3 locus by dephosphorylating Dnmt1 and enhancing Akt phosphorylation at Ser473/Thr308, leading to impaired FoxP3 expression in T_reg cells. In summary, our study proposes Pp6 as a critical positive regulator of FoxP3 that acts by decreasing DNA methylation of the FoxP3 gene enhancer and inhibiting Akt signaling, thus maintaining T_reg cell stability and preventing autoimmune diseases.