Acute lung injury and adult respiratory distress syndrome are characterized by pathological efflux of serum proteins and fluid from capillaries to interstitial tissues and represents a significant complication in the clinical setting with no mitigating therapy. The scope of this research is to lay the foundation for a polymer-based therapy, delivered to inflamed vascular endothelium, that strengthens the capillary barrier. The research spans the use of two polymer prototypes. The first polymer prototype, a tetramethylammonium chloride copolymer with a poly-N-(2-hydroxypropyl)methacrylamide (P-HPMA) backbone that binds to endothelium via ionic interactions, is used to show the efficacy of copolymer delivery to highly permeable endothelium. The second polymer prototype retains the HPMA backbone but exchanges cationic side chains for a peptide that targets E-selectin, an endothelial protein that is upregulated during inflammation and mediates leukocyte tethering and rolling. In this work, we are able to show a significant reduction in solute and solvent flux across endothelial monolayers under cytokine-mediated or mechanical stress in the presence of copolymers. We determined that the attenuation of barrier permeability was due to the polymer’s inhibition of cell signaling mechanism(s) and not only a result of an increased physical barrier. The administration of copolymers attenuates capillary permeability in an ex vivo lung model to demonstrate the viability of targeted copolymer delivery to vascular endothelium as a treatment for vascular permeability and acute lung injury.