Regulatory T cells (T_reg cells) suppress autoreactive immune responses and limit the efficacy of tumor vaccines; however, it remains a challenge to selectively eliminate or inhibit T_reg cells. In this study, the zinc-finger A20, a negative regulator of the Toll-like receptor and tumor necrosis factor receptor signaling pathways, was found to play a crucial part in controlling the maturation, cytokine production and immunostimulatory potency of dendritic cells (DCs). A20-silenced DCs showed spontaneous and enhanced expression of costimulatory molecules and proinflammatory cytokines and had different effects on T cell subsets: they inhibited T_reg cells and hyperactivated tumor-infiltrating cytotoxic T lymphocytes and T helper cells that produced interleukin-6 and tumor necrosis factor-α and were refractory to T_reg cell–mediated suppression. Hence, this study identifies A20 as an antigen presentation attenuator in control of antitumor immune responses during both the priming and the effector phases and provides a strategy to overcome T_reg cell–mediated suppression in an antigen-specific manner, reducing the need to directly target T_reg cells.