Balance in signal transducer and activator of transcription (STAT) activation is a key factor in regulating the fate of naive CD4+ T cells. Here, we demonstrate that AT-rich interactive domain-containing protein 5a (Arid5a) in T cells directs naive CD4+ T cells to differentiate into inflammatory CD4+ T cells, especially Th17 cells, through selective stabilization of Stat3 (but not Stat1 and Stat5) mRNA in an IL-6–dependent manner. Loss of Arid5a in T cells led to reduction of STAT3 level under Th17-polarizing conditions, whereas STAT1 and STAT5 in Arid5a-deficient T cells were highly activated compared with those of WT T cells under the same conditions. These cells displayed the feature of antiinflammatory (Il10-expressing) CD4+ T cells. Thus, we show a T cell–intrinsic role of Arid5a on fate decisions of naive CD4+ T cells through selective stabilization of Stat3 mRNA.