Blimp-1 expression in T cells extinguishes the fate of T follicular helper cells, drives terminal differentiation, and limits autoimmunity. Although various factors have been described to control Blimp-1 expression in T cells, little is known about what regulates Blimp-1 expression in T helper 2 (TH2) cells and the molecular basis of its actions. We report that signal transducer and activator of transcription 3 (STAT3) unexpectedly played a critical role in regulating Blimp-1 in TH2 cells. Furthermore, we found that the cytokine interleukin-10 (IL-10) acted directly on TH2 cells and was necessary and sufficient to induce optimal Blimp-1 expression through STAT3. Together, Blimp-1 and STAT3 amplified IL-10 production in TH2 cells, creating a strong autoregulatory loop that enhanced Blimp-1 expression. Increased Blimp-1 in T cells antagonized STAT5-regulated cell cycle and antiapoptotic genes to limit cell expansion. These data elucidate the signals required for Blimp-1 expression in TH2 cells and reveal an unexpected mechanism of action of IL-10 in T cells, providing insights into the molecular underpinning by which Blimp-1 constrains T cell expansion to limit autoimmunity.