Application of regulatory T cells (Tregs) in transplantation, autoimmunity, and allergy has been extensively explored, but how Foxp3 and Treg stability is regulated in vivo is incompletely understood. Here, we identify a requirement for Deltex1 (DTX1), a contributor to T cell anergy and Foxp3 protein level maintenance in vivo. Dtx1-/- Tregs are as effective as WT Tregs in the inhibition of CD4+CD25- T cell activation in vitro. However, the suppressive ability of Dtx1-/- Tregs is greatly impaired in vivo. We find that Foxp3 expression is diminished when Dtx1-/- Tregs are co-transferred with effector T cells in vivo. DTX1 promotes the degradation of HIF-1. Knockout of HIF-1 restores the Foxp3 stability and rescues the defective suppressive activity in Dtx1-/- Treg cells in vivo. Our results suggest that DTX1 exerts another level of control on Treg stability in vivo by sustaining the expression of Foxp3 protein in Tregs.