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Retinoic acid-related orphan receptor γt (RORγt) is a key master regulator of the differentiation and activation of IL-17 producing CD4 + Th17, CD8 + Tc17 and IL-17/IFN-γ co-producing cells (Th1/17 cells). These cells play critical roles in the pathogenesis of autoimmune diseases such as inflammatory bowel disease and multiple sclerosis. Thus, RORγt is an attractive target for the treatment of these diseases. We discovered TAK-828F, an orally available potent and selective RORγt inverse agonist. The inhibitory effect on the activation and differentiation of Th17 cells by TAK-828F was evaluated in mouse and human primary cells. TAK-828F inhibited IL-17 production from mouse splenocytes and human peripheral blood mononuclear cells dose-dependently at concentrations of 0.01–10 μM without affecting the production of IFN-γ. Additionally, TAK-828F strongly inhibited Th17, Tc17 and Th1/17 cells’ differentiation from naive T cells and memory CD4 + T cells at 100 nM without affecting Th1 cells’ differentiation. In addition, TAK-828F improved Th17/Treg cells’ population ratio by inhibiting Th17 cells’ differentiation and up-regulating Treg cells. Furthermore, TAK-828F, at 100 nM, reduced the production of Th17-related cytokines (IL-17, IL-17F and IL-22) without affecting IFN-γ production in whole blood. These results demonstrate that TAK-828F has the potent and selective inhibitory activity against RORγt both in mouse and human cells. Additionally, oral administration of TAK-828F showed promising efficacy in naive T cell transfer mouse colitis model. TAK-828F may provide a novel therapeutic option to treat immune diseases by inhibiting Th17 and Th1/17 cells’ differentiation and improving imbalance between Th17 and Treg cells.