Distinct CD4⁺ T cell subsets are critical for host defense and immunoregulation. Although these subsets can act as terminally differentiated lineages, they have been increasingly noted to demonstrated plasticity. MicroRNAs are factors that control T cell stability and plasticity. Here we report that naturally occurring regulatory T cells (T_reg cells) had high expression of the microRNA miR-10a and that miR-10a was induced by retinoic acid and transforming growth factor-β (TGF-β) in inducible T_reg cells. By simultaneously targeting the transcriptional repressor Bcl-6 and the corepressor Ncor2, miR-10a attenuated the phenotypic conversion of inducible T_reg cells into follicular helper T cells. We also found that miR-10a limited differentiation into the T_H17 subset of helper T cells and therefore represents a factor that can fine-tune the plasticity and fate of helper T cells.