Fouling is the undesirable accumulation of a material on a wide variety of objects and has now become a widespread global problem from land to ocean with both economic and environmental penalties. Here, we report protein-repellent properties of ultrathin polymer films that are considered to be of structural origin and generalizable across homopolymer systems. Ultrathin polymer films composed of polystyrene, poly(2-vinyl pyridine), poly(methyl methacrylate), and polybutadiene with different thicknesses (h) ranging from 2 to 60 nm were prepared on silicon substrates. Bovine serum albumin and fibrinogen (both are fluorescein-labeled) were used as model proteins. The polymer thin films were incubated in the protein solution, removed, and then rinsed with water. The fluorescence intensity I(h) measured by a photon-counting spectrofluorometer generated a master curve over the film thickness regardless of the polymer and protein choice, revealing the two different protein adsorption regimes with the following thicknesses: (i) below the critical thickness (h(c) congruent to 20 nm), I(h) is minimal (nearly zero at h < 5 nm) and exhibits a very weak thickness dependence; (ii) at h > h(c), I(h) exhibits very strong thickness dependence (I(h) similar to h(2)). Molecular dynamics simulations identified a positive correlation between protein adsorption and highly packed conformations of polymer chains, which either adsorb on the substrate or do not adsorb but are in contact with the adsorbed polymer chains, resulting in a protein-repellent "dense layer" at h < h(c). In addition, the effect of the dense layer propagates into the film interior up to at least 60 nm, resulting in an "interphase" that shows the quadratic adsorption behavior. These experimental and computational findings detail a new mechanism behind the structure-driven protein-repellent properties of polymer structures under nanoconfinement over surface chemistry.