T-cell activation RhoGTPase-activating protein (TAGAP) is a GTPase-activating protein specific for RhoA that is exclusively expressed in activated T cells. Genome-wide association studies and metagenome SNPs analyses have indicated that TAGAP is associated with the pathogenesis of multiple autoimmune diseases, including psoriasis, rheumatoid arthritis, Crohn's disease, celiac disease and multiple sclerosis. However, the precise function of TAGAP remains unclear. Because TH17 cells contribute to TAGAP-associated autoimmune diseases, we hypothesized that TAGAP plays key roles in the differentiation and/or function of TH17 cells. To evaluate this hypothesis, we analyzed the effect of TAGAP on TH17 differentiation in vitro and established a line of TAGAP-deficient mice. We found that TAGAP was required for TH17 differentiation in vitro and that the loss of TAGAP in mice ameliorated the clinical features of experimental autoimmune encephalomyelitis, indicating that TAGAP is critical for disease progression. We also demonstrated that TAGAP interacts with RhoH, an adapter protein that interacts with lck and ZAP70 in proximal TCR signaling. TAGAP competes with ZAP70 for RhoH binding, thereby inhibiting TCR-associated signal transduction. Consistent with these findings, TCR-induced ERK activation was increased in TAGAP-deficient T cells. Because the upregulation of TCR signaling inhibits Th17 differentiation, TAGAP may prevent TCR signaling activity from reaching the limit of the induction of TH17 cells. Collectively, our findings indicate that TAGAP is a novel factor required for TH17-cell differentiation and that TAGAP potentially represents a novel target of autoimmune disease therapies.