Plasma surface modification of glass fibers (GFs) is an alternative technology to wet chemical processes employed for commercial sizing used for GF-reinforced polymer composites. The sizing (functional coating) must improve compatibility and form a strong but tough link between the fiber and the polymer matrix. However, commercial sizing is a heterogeneous coating of variable thickness, with only a small portion of the total sizing chemically bonded to the fiber surface. In addition, the bonding is hydrolytically unstable. Plasma modification of GFs enables the deposition of a homogeneous film (interlayer) of controlled thickness and controlled chemical, mechanical, and surface properties. The ability to control the elemental composition, chemical structure (functional groups), wettability, surface roughness, interfacial adhesion, and Young’s modulus of deposited films in relatively wide ranges allows optimization of the interlayer for a specific composite system, i.e., the fiber and the matrix. With improved interfacial adhesion and higher cross-linking of the plasma polymer network, the plasma coatings have greater hydrolytic stability compared to commercial sizing. Optimized plasma modification of GFs enabled a 32% increase in the shear strength of GF/polyester composite compared to commercially sized fibers. Plasma modification is a prospective technology for the construction of more complex film nanostructures, such as the multilayer and gradient interlayers required for the novel conception of composites without interfaces.