Polydopamine (PDA) is known for its versatile adhesion, yet traditional air-oxidation methods are slow and often yield uneven coatings. In this study, we employed a CuSO₄/H₂O₂-triggered oxidation system to rapidly generate thin, uniform PDA layers on polymer substrates. To address the inherent flammability of polymers without compromising their bulk mechanical properties, we functionalized this PDA layer with a DOPO-based phosphorus-nitrogen flame retardant. The PDA coating serves as a reactive platform, enabling the covalent attachment of DOPO derivatives. The functionalized coating provides a dual-phase flame-retardant mechanism: it releases radical scavengers (PO- ) in the gas phase to interrupt combustion and promotes robust char formation in the condensed phase.