Increased endothelial permeability contributes to the morbidity and mortality associated with chronic inflammatory diseases, including acute lung injury. Cyclic AMP response element-binding protein (CREB) transcriptional factor induces genes that regulate inflammation and vascular remodeling. However, the role of CREB in regulating endothelial barrier function is unknown. Here, we demonstrate that CREB maintains basal endothelial barrier function and suppresses endothelial permeability increase by diverse agonists such as thrombin, lipopolysaccharide, histamine, and VEGF. We show that CREB transcriptionally controls the expression of p190RhoGAP-A, a GTPase-activating protein that inhibits small GTPase RhoA. Impairing CREB function using small interfering RNA or dominant-negative (dn)–CREB mutant (dn-CREB) markedly suppressed p190RhoGAP-A expression, increased RhoA activity, induced actin stress fiber formation, and produced an amplified and protracted increase in endothelial permeability in response to thrombin. Rescuing p190RhoGAP-A expression restored the permeability defect in dn-CREB–transducing endothelial cells. These findings were recapitulated in vivo because dn-CREB expression in mice vasculature increased basal lung microvessel permeability and exaggerated permeability increase induced by thrombin and lipopolysaccharide. Inhibiting RhoA signaling restored endothelial barrier dysfunction in the dn-CREB–expressing lung microvasculature. These results uncover a pivotal role of CREB in regulating endothelial barrier function by restricting RhoA signaling through controlling p190RhoGAP-A expression.