Circulating levels of high-density lipoprotein (HDL) cholesterol are inversely related to the risk of cardiovascular disease, and HDL and the HDL receptor scavenger receptor class B type I (SR-BI) initiate signaling in endothelium through src that promotes endothelial NO synthase activity and cell migration. Such signaling requires the C-terminal PDZ-interacting domain of SR-BI. Here we show that the PDZ domain–containing protein PDZK1 is expressed in endothelium and required for HDL activation of endothelial NO synthase and cell migration; in contrast, endothelial cell responses to other stimuli, including vascular endothelial growth factor, are PDZK1-independent. Coimmunoprecipitation experiments reveal that Src interacts with SR-BI, and this process is PDZK1-independent. PDZK1 also does not regulate SR-BI abundance or plasma membrane localization in endothelium or HDL binding or cholesterol efflux. Alternatively, PDZK1 is required for HDL/SR-BI to induce Src phosphorylation. Paralleling the in vitro findings, carotid artery reendothelialization following perivascular electric injury is absent in PDZK1^-/- mice, and this phenotype persists in PDZK1^-/- mice with genetic reconstitution of PDZK1 expression in liver, where PDZK1 modifies SR-BI abundance. Thus, PDZK1 is uniquely required for HDL/SR-BI signaling in endothelium, and through these mechanisms, it is critically involved in the maintenance of endothelial monolayer integrity.