Selective JAK3 inhibitors have been shown to have a potential benefit in the treatment of autoimmune disorders. Here we report the identification of a series of pyrazolopyrimidine derivatives as potent JAK3 inhibitors that exploit a unique cysteine (Cys909) residue in JAK3. Most of these compounds (13k, 13n and 13 t), displayed stronger anti-JAK3 kinase activity and selectivity than tofacitinib. Furthermore, the most active inhibitor 13t (IC = 0.1 nM), also exhibited favourable selectivity for JAK3 in a panel of 9 kinases which contain the same cysteine. In a series of cytokinestimulated cellular analysis, compound 13 t, could potently block the JAK3-STAT signaling pathway. Further biological studies, including cellular antiproliferative activity assays and a rat adjuvant-induced arthritis model for in vivo evaluation, also indicated its efficacy and low toxicity in the treatment of rheumatoid arthritis. The results of these experimental explorations suggested that 13t is a promising lead compound for the development of selective JAK3 inhibitor with therapeutic potential in rheumatoid arthritis.