Extracellular nucleotides are pleiotropic regulators of mammalian cell function. Adenosine triphosphate (ATP) released from CD4⁺ helper T cells upon stimulation of the T cell receptor (TCR) contributes in an autocrine manner to the activation of mitogen-activated protein kinase (MAPK) signaling through purinergic P2X receptors. Increased expression of p2rx7, which encodes the purinergic receptor P2X7, is part of the transcriptional signature of immunosuppressive CD4⁺CD25⁺ regulatory T cells (T_regs). Here, we show that the activation of P2X7 by ATP inhibits the suppressive potential and stability of T_regs. The inflammatory cytokine interleukin-6 (IL-6) increased ATP synthesis and P2X7-mediated signaling in T_regs, which induced their conversion to IL-17–secreting T helper 17 (T_H17) effector cells in vivo. Moreover, pharmacological antagonism of P2X receptors promoted the cell-autonomous conversion of naïve CD4⁺ T cells into T_regs after TCR stimulation. Thus, ATP acts as an autocrine factor that integrates stimuli from the microenvironment and cellular energetics to tune the developmental and immunosuppressive program of the T cell in adaptive immune responses.