A subpopulation (60-70%) of Foxp3(+) regulatory T cells (Tregs) in both mouse and man expresses the transcription factor Helios, but its role in Treg function is still unknown. We generated Treg-specific Helios-deficient mice to examine the function of Helios in Tregs. We show that the selective deletion of Helios in Tregs leads to slow, progressive systemic immune activation, hypergammaglobulinemia, and enhanced germinal center formation in the absence of organ-specific autoimmunity. Helios-deficient Treg suppressor function was normal in vitro, as well as in an in vivo inflammatory bowel disease model. However, Helios-deficient Tregs failed to control the expansion of pathogenic T cells derived from scurfy mice, failed to mediate T follicular regulatory cell function, and failed to control both T follicular helper cell and Th1 effector cell responses. In competitive settings, Helios-deficient Tregs, particularly effector Tregs, were at a disadvantage, indicating that Helios regulates effector Treg fitness. Thus, we demonstrate that Helios controls certain aspects of Treg-suppressive function, differentiation, and survival.