Regulatory T (T_reg) cells are critical in preventing aberrant immune responses. Posttranscriptional control of gene expression by microRNA (miRNA) has recently emerged as an essential genetic element for T_reg cell function. Here, we report that mice with T_reg cell–specific ablation of miR-142 (hereafter Foxp3^CremiR-142^fl/fl mice) developed a fatal systemic autoimmune disorder due to a breakdown in peripheral T-cell tolerance. Foxp3^CremiR-142^fl/fl mice displayed a significant decrease in the abundance and suppressive capacity of T_reg cells. Expression profiling of miR-142–deficient T_reg cells revealed an up-regulation of multiple genes in the interferon gamma (IFNγ) signaling network. We identified several of these IFNγ-associated genes as direct miR-142-3p targets and observed excessive IFNγ production and signaling in miR-142–deficient T_reg cells. Ifng ablation rescued the T_reg cell homeostatic defect and alleviated development of autoimmunity in Foxp3^CremiR-142^fl/fl mice. Thus, our findings implicate miR-142 as an indispensable regulator of T_reg cell homeostasis that exerts its function by attenuating IFNγ responses.