In the present study, we aimed to investigate the immunosuppressive activity of jaceosidin, a flavone isolated from Artemisia vestita, on T lymphocytes both in vitro and in vivo, and further explore its potential molecular mechanism. Jaceosidin exerted a significant inhibition on the T cell proliferation and activation induced by concanavalin A (Con A) in a concentration-dependent manner and it also inhibited the secretion of the proinflammatory cytokines such as IL-2, TNF-α and IFN-γ of activated T cells. Further study showed that jaceosidin down-regulated STAT1 activation and T-bet expression in activated T cells. Moreover, in order to investigate the immunosuppressive effect of jaceosidin in vivo, the picryl chloride (PCl)-induced ear contact dermatitis model was performed on BALB/c mice. Jaceosidin significantly ameliorated PCl-induced ear swelling in a dose-dependent manner, which was due to its inhibition of the STAT1/T-bet signaling pathway. In summary, these findings suggest that jaceosidin exerts its immunosuppressive effect both in vitro and in vivo through inhibiting T cell proliferation and activation, which is closely associated with its potent downregulation of the IFN-γ/STAT1/T-bet signaling pathway.