Lignin, a renewable aromatic biopolymer abundantly found in nature, has attracted increasing attention as a sustainable flame retardant additive owing to its high carbon content, intrinsic thermal stability, and strong charring ability. This review provides a comprehensive overview of the structural and thermal characteristics of lignin and highlights key chemical modification strategies—including phosphorus-, nitrogen-, and siliconbased functionalization as well as synergistic multi-element modification-to enhance its flame retardant performance. Representative flame retardant evaluation techniques, such as thermogravimetric analysis (TGA), limiting oxygen index (LOI), UL-94 vertical burning tests, and cone calorimetry, are introduced with emphasis on their relevance to condensed-phase and gas-phase flame retardant mechanisms. The incorporation of ligninbased flame retardant additives into thermoplastic and thermosetting polymer composites is discussed in terms of flammability suppression, mechanical performance retention, and interfacial compatibility. Key challenges, including dispersion stability, processability at high additive loadings, and compatibility with polymer matrices, are critically addressed. Finally, recent advances in lignin-derived flame retardant systems are analyzed, and future perspectives toward the development of commercially viable, eco-friendly flame retardant additives are proposed. This review aims to support material innovation in flame-retardant polymer composites through the strategic valorization of lignin as a multifunctional and sustainable bio-based additive.