Vascular-targeted drug carriers must localize to the wall (i.e., marginate) and adhere to a diseased endothelium to achieve clinical utility. The particle size has been reported as a critical physical property prescribing particle margination in vitro and in vivo blood flows. Different transport process steps yield conflicting requirements—microparticles are optimal for margination, but nanoparticles are better for intracellular or tissue delivery. Here, we evaluate deformable hydrogel microparticles as carriers for transporting nanoparticles to a diseased vascular wall. Depending on microparticle modulus, nanoparticle-loaded poly(ethylene glycol)–based hydrogel microparticles delivered significantly more 50-nm nanoparticles to the vessel wall than freely injected nanoparticles alone, resulting in >3000% delivery increase. This work demonstrates the benefit of optimizing microparticles’ efficient margination to enhance nanocarriers’ transport to the vascular wall.