Forkhead box P3 (Foxp3)⁺ T regulatory (T_reg) cells maintain immune homeostasis and limit autoimmunity but can also curtail host immune responses to various types of tumors^,. Foxp3⁺ T_reg cells are therefore considered promising targets to enhance antitumor immunity, and approaches for their therapeutic modulation are being developed. However, although studies showing that experimentally depleting Foxp3⁺ T_reg cells can enhance antitumor responses provide proof of principle, these studies lack clear translational potential and have various shortcomings. Histone/protein acetyltransferases (HATs) promote chromatin accessibility, gene transcription and the function of multiple transcription factors and nonhistone proteins^,. We now report that conditional deletion or pharmacologic inhibition of one HAT, p300 (also known as Ep300 or KAT3B), in Foxp3⁺ T_reg cells increased T cell receptor–induced apoptosis in T_reg cells, impaired T_reg cell suppressive function and peripheral T_reg cell induction, and limited tumor growth in immunocompetent but not in immunodeficient mice. Our data thereby demonstrate that p300 is important for Foxp3⁺ T_reg cell function and homeostasis in vivo and in vitro, and identify mechanisms by which appropriate small-molecule inhibitors can diminish T_reg cell function without overtly impairing T effector cell responses or inducing autoimmunity. Collectively, these data suggest a new approach for cancer immunotherapy.