Th17 cells, which have been implicated in autoimmune diseases including rheumatoid arthritis (RA), require the JA1(-STAT3 pathway for their differentiation and functions. Recently, JAK inhibitors have been developed as a therapeutic drug for RA. However, the current JAR inhibitors are not optimized to STAT3 compared with other STATs. In this study, we found a new lead compound of a small molecule JAR-STAT inhibitor, 2-[(3-Carbamoyl-2-thienyl)amino]-2-oxoethyl (2,6-dichlorophenyl)acetate, which inhibits STAT3 as efficiently as other STATs. This compound, named JI069, was selected by STAT3 reporter assay in combination with an in silico docking model. JI069 inhibited gp130 signaling by inducing dissociation between gp130 and JAK1. In HEK293T cells and primary T cells, JI069 suppressed STAT3 activation as efficiently as other STATs, including STAT1, STAT5, and STAT6. JI069 effectively suppressed Th1, Th2, and Th17 differentiation while strongly promoted iTreg differentiation. JI069 suppressed symptoms of the collagen-induced arthritis (CIA) model in mice, and inhibited the cytokine production from T cells as well as the STAT3 phosphorylation of synovial cells. These data suggest that JI069 is a new type of JAR inhibitor which has potential for the treatment of immunological disorders. (C) 2015 Elsevier Inc. All rights reserved.