Phlorotannins are a specialized class of polyphenolic compounds that occur solely in brown algae (Phaeophyceae). These compounds have garnered significant interest due to their diverse range of biological activities, including antioxidant, anti-inflammatory, and antimicrobial properties. As secondary metabolites, phlorotannins play a crucial role in the defense mechanisms of brown algae, protecting them from herbivory and environmental stresses. Recent studies have explored their potential applications in pharmaceuticals, nutraceuticals, and cosmeceuticals, highlighting their importance in both ecological and industrial contexts. This discussion will delve into the chemical structure, biosynthesis, ecological functions, and potential applications of phlorotannins, providing a comprehensive overview of their significance in both natural and applied sciences.