<p>Bio-based 2,5-furandicarboxylic acid polyesters offer significant promise for reducing energy and environmental crises. However, their intrinsic flammability remains a critical limitation, and conventional flame-retardant strategies often compromise their mechanical properties, hindering their practical applications. Herein, a 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-based comonomer (DDP) was used to synthesize flame-retardant poly(ethylene furandicarboxylate-<i>co</i>-phosphaphenanthrene) (PEFDn). The covalent integration of DDP confers intrinsic flame retardancy, avoiding the plasticization and migration issues associated with additive-type systems. Upon thermal decomposition, the DOPO-derived moieties release phosphoric acid and radical scavengers, promoting char formation and suppressing flame propagation. Furthermore, density functional theory (DFT) calculations combined with non-covalent interaction (NCI) analysis revealed that DOPO dimer molecules adopt a stable parallel-displaced <i>π–π</i> stacking configuration, potentially facilitating microphase separation and enhancing the energy dissipation capability. PEFD₁₀ achieves a UL-94 V-0 rating while simultaneously increasing impact toughness from 1.5 kJ/m² to 14.7 kJ/m². Importantly, PEFDn maintained acceptable oxygen-barrier properties. PEFD₁₀ also exhibited high transparency and UV-shielding performance. The combination of intrinsic flame safety, impact-toughness resistance, UV shielding, and an oxygen barrier ensures reliable protection of electrical components and long-term operational stability. The integration of multiple critical properties within a single bio-based material represents a novel approach for enabling sustainable polymer solutions for high-performance electrical applications.</p>