Surface functionalization is a key process in rendering various materials biocompatible. Whereas a number of techniques and technologies have been developed for the purpose of biofunctionalization, plasma treatment enables highly efficient surface modification. Extending plasma treatment to biomolecules in the liquid phase will control biofunctionalization via a simple process. However, interactions between plasma discharge and biomolecules or solvents are poorly understood, potentially leading to the technical limitation as to the utility of plasma treatment. In this study, we developed a technology for substrate biofunctionalization that does not require surface modification but involves direct treatment of a collagen molecules with liquid-phase plasma discharge. Biofunctionalization of collagen by plasma treatment comprises three processes that increase its reactivity with hydrophobic substrates: (1) charge-dependent changes in surface and interfacial properties of the collagen solution; (2) local conformational changes of the collagen molecules without their global structural alterations; and (3) induction of a micelle-like association formed by collagen molecules. We anticipate such plasma-induced functionalization of protein molecules to provide a versatile technique in the applications of biomaterials, including those related to pharmaceuticals and cosmetics.