Cardiovascular diseases are the leading causes of death in the developed world. Atherosclerosis, a type of cardiovascular disease where blood vessels stiffen and accumulate lipid-laden plaques that restrict blood flow, is a major underlying cause of many cardiovascular events, including heart attack or stroke. The stiffening of blood vessels occurs also occurs with age, which is considered a significant risk factor for atherosclerosis. Although a relationship exists between age, vessel stiffening, and atherosclerosis, the exact mechanisms underlying this relationship are not wellcharacterized. Herein, we demonstrate that age-related vessel stiffening promotes hallmarks of atherosclerosis, specifically endothelial permeability and vascular smooth muscle cell (VSMC) motility. Endothelial cells cultured on synthetic hydrogel substrates made to mimic the stiffness of healthy and diseased vessels exhibited increased permeability on stiffer substrates. This increase in permeability is due to the widening of endothelial cell-cell junctions and results in increased leukocyte transmigration through endothelial monolayers. Using a mouse model of aging, aged mice were also found to display increased vessel permeability and junction separation compared to young mice. Substrate stiffness induces endothelial permeability and junctional separation by upregulating cell contractility in a RhoGTPase-dependent manner. Inhibition of cell contractility pharmacologically or with siRNA restored endothelial monolayer integrity in vitro and in vivo. In the later stages of atherosclerosis, VSMCs migrate into the intimal layer from the medial layer of the artery and contribute to the formation of atherosclerotic plaques. Age-related vessel stiffening mediates this migration, which is also stimulated by the release of platelet-derived growth factor (PDGF) by endothelial cells or entrapped inflammatory cells. PDGF also induces the formation of circular dorsal ruffles (CDRs), actin-based structures associated with enhanced cell motility. Herein, our data demonstrate that vessel stiffening promotes the formation of CDRs by increasing cell contractility and intracellular pre-stress. When VSMC contractility was enhanced or inhibited pharmacologically, cells significantly increased or decreased CDR formation, respectively. Together, our results suggest that extracellular matrix stiffening alone, which occurs during aging, can lead to endothelial monolayer disruption, VSMC motility, and atherosclerosis pathogenesis. Our data suggest that targeting vascular stiffening and/or cellular responses to stiffening may be a promising approach to preventing atherosclerosis.