In this study, we developed a substrate-independent initiator film that can undergo surface-initiated polymerization to form an antifouling brush. Inspired by the melanogenesis found in nature, we synthesized a tyrosine-conjugated bromide initiator (Tyr-Br) that contains phenolic amine groups as the dormant coating precursor and α-bromoisobutyryl groups as the initiator. The resultant Tyr-Br was stable under ambient air conditions and underwent melanin-like oxidation only in the presence of tyrosinase to form an initiator film on various substrates. Subsequently, an antifouling polymer brush was formed using air-tolerant activators regenerated by electron transfer for atom transfer radical polymerization (ARGET ATRP) of zwitterionic carboxybetaine. The entire surface coating procedure, including the initiator layer formation and ARGET ATRP, occurred under aqueous conditions and did not require organic solvents or chemical oxidants. Therefore, antifouling polymer brushes can be feasibly formed not only on experimentally preferred substrates (e.g., Au, SiO2, and TiO2) but also on polymeric substrates such as poly(ethylene terephthalate) (PET), cyclic olefin copolymer (COC), and nylon.