The γ-amino butyric acid (GABA)ergic system shapes the activation and function of immune cells. The present study was conducted to explore the regulation of GABA transporter (GAT)-2 on the differentiation of Th17 cells. Here we found that Th17 cells show higher abundance of GAT-2, and have distinct cellular metabolic signatures, such as the GABA shunt pathway, as compared to naïve T cells. GAT-2 deficiency had little effect on the metabolic signature in naïve T cells, but impaired the GABA uptake and GABA shunt pathway in Th17 cells. GAT-2 deficiency had little effect on T cell development and peripheral T cell homeostasis; however, its deficiency promoted Th17 cell differentiation in vitro. Mechanistically, GAT-2 deficiency promoted differentiation of Th17 cells through activation of GABA-mTOR signaling. In a mouse model of intestinal infection and inflammation, GAT-2 deficiency promoted Th17 responses. Collectively, GAT-2 deficiency promotes Th17 cell responses through activation of GABA-mTOR signaling.