microRNAs (miRNA) are essential for regulatory T cell (Treg) function but little is known about the functional relevance of individual miRNA loci. We identified the miR-17–92 cluster as CD28 costimulation dependent, suggesting that it may be key for Treg development and function. Although overall immune homeostasis was maintained in mice with miR-17–92–deficient Tregs, expression of the miR-17–92 miRNA cluster was critical for Treg accumulation and function during an acute organ-specific autoimmune disease in vivo. Treg-specific loss of miR-17–92 expression resulted in exacerbated experimental autoimmune encephalitis and failure to establish clinical remission. Using peptide-MHC tetramers, we demonstrate that the miR-17–92 cluster was specifically required for the accumulation of activated Ag-specific Treg and for differentiation into IL-10–producing effector Treg.