The differentiation and function of IL-17–producing Th17 cells are tightly regulated by specific transcription factors and cytokines, which are the key participants in the pathogenesis of multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE). Although specific miRNAs have been shown to be involved in the development of MS and EAE, the potential role of miRNAs in the context of Th17-driven autoimmunity is just beginning to be clarified. miR-20b has been reported as a downregulated miRNA in blood cells of MS patients. In this report, it was further studied in greater detail because we found it was significantly downregulated during EAE, and, in the in vitro differentiation model, Th17 cells had lower expression of miR-20b than did Th1, Th2, or inducible T regulatory cells. Ectopic expression of miR-20b repressed Th17 differentiation in vitro. Using lentiviral vectors for miR-20b overexpression in vivo, we demonstrated that overexpression of miR-20b led to decreased Th17 cells and reduced severity of EAE. Furthermore, we also identified both RAR-related orphan receptor γt and STAT3 as potential targets of miR-20b. Finally, we confirmed that the mild disease severity and low number of Th17 cells in LV-miR-20b–infected mice were largely reversed by coinfection of these mice with lentivirus-expressing RAR-related orphan receptor γt or STAT3 3′-untranslated regions. Taken together, our results contribute to the importance of miRNAs in Th17 differentiation and pathogenesis of MS and EAE.