Distinct classes of protective immunity are guided by activation of STAT transcription factor family members in response to environmental cues. CD4⁺ regulatory T cells (T_regs) suppress excessive immune responses, and their deficiency results in a lethal, multi-organ autoimmune syndrome characterized by T helper 1 (T_H1) and T helper 2 (T_H2) CD4⁺ T cell–dominated lesions. Here we show that pathogenic T_H17 responses in mice are also restrained by T_regs. This suppression was lost upon T_reg-specific ablation of Stat3, a transcription factor critical for T_H17 differentiation, and resulted in the development of a fatal intestinal inflammation. These findings suggest that T_regs adapt to their environment by engaging distinct effector response–specific suppression modalities upon activation of STAT proteins that direct the corresponding class of the immune response.