High mobility group box 1 (HMGB1) protein is a crucial nuclear cytokine that elicits severe vascular inflammatory diseases. Oenanthe javanica (water dropwort) extract has anti-arrhythmic, neuroprotective and anti-diabetic activity. However, isorhamnetin-3-O-galactoside (I3G), an active compound from O. javanica, is not researched well for its biological activity. Here, we investigated the anti-inflammatory activities of I3G by monitoring the effects of I3G on the lipopolysaccharide (LPS) or cecal ligation and puncture (CLP)-mediated release of HMGB1 and HMGB1 or CLP-mediated modulation of inflammatory responses. I3G potently inhibited the release of HMGB1 and down-regulated HMGB1-dependent inflammatory responses in human endothelial cells. I3G also inhibited HMGB1-mediated hyperpermeability and leukocyte migration in mice. Further studies revealed that I3G suppressed the production of tumor necrosis factor-α and activation of nuclear factor-κB by HMGB1. In addition, I3G reduced CLP-induced HMGB1 release and sepsis-related mortality. Given these results, I3G should be viewed as a candidate therapeutic agent for the treatment of severe vascular inflammatory diseases such as sepsis or septic shock via inhibition of the HMGB1 signaling pathway. J. Cell. Biochem. 114: 336–345, 2013. © 2012 Wiley Periodicals, Inc.