Plants are rich sources of lignin, a complex aromatic polymer that shows great potential as a natural antioxidant. Its structure, which consists of aromatic rings possessing multiple functional groups such as hydroxyl and methoxyl groups, enables it to scavenge free radicals and provide an antioxidant property. However, several factors, including the source plant, the extraction process, and the lignin’s unique structure, can affect the efficiency of this natural antioxidant. This chapter delves into the realm of naturally occurring antioxidants, approaches to determining their activity, and the exceptional instance of lignin serving as a natural antioxidant. The correlation between lignin’s structure, its capacity to scavenge free radicals, and the several factors that affect its antioxidant properties are discussed in detail. Finally, the chapter explores the fascinating prospective applications of lignin as an antioxidant in food, health, and material fields.