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Summary: Foxp3+ regulatory T cells (Treg) are essential modulators of immune responses, but the molecular mechanisms underlying their function are not fully understood. Here we show that the transcription factor Blimp-1 is a crucial regulator of the Foxp3+RORγt+ Treg subset. The intrinsic expression of Blimp-1 in these cells is required to prevent production of Th17-associated cytokines. Direct binding of Blimp-1 to the Il17 locus in Treg is associated with inhibitory histone modifications but unaltered binding of RORγt. In the absence of Blimp-1, the Il17 locus is activated, with increased occupancy of the co-activator p300 and abundant binding of the transcriptional regulator IRF4, which is required, along with RORγt, for IL-17 expression in the absence of Blimp-1. We also show that despite their sustained expression of Foxp3, Blimp-1−/− RORγt+IL-17-producing Treg lose suppressor function and can promote intestinal inflammation, indicating that repression of Th17-associated cytokines by Blimp-1 is a crucial requirement for RORγt+ Treg function. : Ogawa et al. demonstrate that the transcription factor Blimp-1 is required to prevent production of Th17-associated cytokines and inflammatory activity of microbiota-specific Foxp3+RORγt+ Treg. These findings uncover a critical role for Blimp-1 in Foxp3+ Treg function and shed light on the intricate mechanisms underlying Treg phenotypic stability. Keywords: Blimp-1, Prdm1, Foxp3, Treg, RORγt, IL-17, IRF4, transcription, Foxp3, intestinal