IL-33 administration is associated with facilitation of Th2 responses and cardioprotective properties in rodent models. However, in heart transplantation, the mechanism by which IL-33, signaling through ST2L (the membrane-bound form of ST2), promotes transplant survival is unclear. We report that IL-33 administration, while facilitating Th2 responses, also increases immunoregulatory myeloid cells and CD4+ Foxp3+ regulatory T cells (Tregs) in mice. IL-33 expands functional myeloid-derived suppressor cells, CD11b+ cells that exhibit intermediate (int) levels of Gr-1 and potent T cell suppressive function. Furthermore, IL-33 administration causes an St2-dependent expansion of suppressive CD4+ Foxp3+ Tregs, including an ST2L+ population. IL-33 monotherapy after fully allogeneic mouse heart transplantation resulted in significant graft prolongation associated with increased Th2-type responses and decreased systemic CD8+ IFN-γ+ cells. Also, despite reducing overall CD3+ cell infiltration of the graft, IL-33 administration markedly increased intragraft Foxp3+ cells. Whereas control graft recipients displayed increases in systemic CD11b+ Gr-1 hi cells, IL-33–treated recipients exhibited increased CD11b+ Gr-1 int cells. Enhanced ST2 expression was observed in the myocardium and endothelium of rejecting allografts, however the therapeutic effect of IL-33 required recipient St2 expression and was dependent on Tregs. These findings reveal a new immunoregulatory property of IL-33. Specifically, in addition to supporting Th2 responses, IL-33 facilitates regulatory cells, particularly functional CD4+ Foxp3+ Tregs that underlie IL-33–mediated cardiac allograft survival.