TNF-receptor superfamily member 25 (TNFRSF25, DR3), as a member of the TNF receptor superfamily, is constitutively expressed on CD4+ T cells at low levels and upregulated after activation. Signaling through TNFRSF25 enhances Th2 and Th17 cytokine production and contributes to the pathology of several autoimmune diseases. Here we investigate the role of TNFRSF25 on CD4+FoxP3+ Treg cells, where it is constitutively expressed at high levels. We show that TNFRSF25 is unique among several TNFRSF members in its ability to expand Treg in vivo to 30-35% of CD4+ T cells. For TNFRSF25 signaling we use an agonistic α-TNFRSF25 antibody, clone 4C12. The expansion of CD4+FoxP3+ cells is due to increased proliferation of CD4+FoxP3+CD25int cells that express high levels of GITR and CD103. 4C12 expanded CD4+FoxP3+ cells retain TGFβ-dependent suppressive activity ex vivo, except in the continued presence of 4C12. Furthermore, 4C12 rescued TL1A-/- mice from susceptibility to dextran sodium sulfate induced colitis. These data highlight the plasticity of the CD4+FoxP3+ regulatory T cell pool and provide first evidence of a physiological pathway leading to Treg-specific expansion in vivo. Therefore, using TNFRSF25 agonists to modulate Treg function may have important applications for the treatment of autoimmune diseases, chronic infection, transplantation and cancer.