Polyethylene (PE) accounts for more than a quarter of all plastic produced, owing to its exceptional material properties and scalable production.¹ However, pristine PE lacks critical properties, such as adhesion, that are needed for many additional commercial applications.² Selective post-polymerization functionalization of PE can impart properties that are absent in the pristine polymer, but global functionalization in a uniform medium perturbs bulk properties.^3–5 Here, we demonstrate catalytic functionalization of solid PE at a solid–liquid interface. The resulting materials possess enhanced surface properties while maintaining bulk properties, such as melting temperature or tensile strength. The method selectively converts methylene units to ketones with just 0.001 mol% iron catalyst below 80 °C with inexpensive oxidants. PE functionalized at the solid–liquid interface adheres to polar surfaces up to 90-fold more strongly than does pristine PE and up to 4-fold more strongly than does PE functionalized in a uniform medium at a comparable level. We demonstrate that the surface ketones can be modified to enhance adhesive strength further and that the augmented surface properties persist for longer than they do after other unselective functionalizations. Thus, selective C–H bond functionalization at the solid–liquid interface produces materials inaccessible by bulk functionalization methods or by traditional surface oxidations.