Metallization is a common method to produce functional or decorative coatings on plastic surfaces. State-of-the-art technologies require energy-intensive process steps and the use of organic solvents or hazardous substances to achieve sufficient adhesion between the polymer and the metal layer. The present study introduces a facile bio-inspired "green" approach to improve this technology: the use of dopamine, a smallmolecule mimic of the main structural component of adhesive mussel proteins, as an adhesion promoter. To understand dopamine adhesion and identify conditions for successful metallization, polyethylene surfaces were dip-coated with dopamine and metallized with nickel by electroless metallization; essential parameters such as temperature, pH value, concentration of dopamine and buffer, and the deposition time were systematically varied. Effects of adding oxidants to the dopamine bath, cross-linking, thermal and UV post-treatment of the polydopamine film, and plasma pretreatment of the substrate were investigated. The properties of the polydopamine layer and the quality of the metal film were studied by physico-chemical, optical, and mechanical techniques. It was shown that simple dip-coating of the substrate with dopamine under optimal conditions is sufficient to support metal layers with a good optical quality. Technologically relevant metal layer quality and adhesion were obtained with annealed and UV-treated polydopamine films and enhanced by plasma pretreatment of the substrate. The study shows that dopamine provides a new interfacial design for plastic metallization that can reduce energy consumption, use of hazardous substances, and reject rate during manufacturing. The results are essential findings for further technological developments of a universal platform to promote adhesion between plastics and metal or potentially also other material classes, enabling economic material development and more eco-friendly applications.