Functionalizing biopolymers via structural modification is a new trend in biopolymer research. Alginates and fucoidans are two of the most renowned biopolymers from marine brown algae with numerous potential applications in engineering and biomedical science due to their desirable physicochemical characteristics and biofunctional properties. Alginates are particularly attractive for its biocompatibility and hydro jelling properties while fucoidans are famous for its broad range of bioactive effects attributed to their sulfate substitution. Structurally modified alginates have shown functional properties different to their native state which is suitable for a variety of potential applications in tissue engineering, wound healing, drug delivery and as fillers and viscosity controlling agents while enhanced biofunctional properties are achieved via structurally modified fucoidans. The present review encloses a comprehensive overview of recent developments in alginates and fucoidans derivatization methods such as controlled degradation, cross-linking, oxidation, substitution of different functional groups and co-polymerizing techniques. An overview of their potential applications and new perspectives for future studies are discussed as an insight into the future developments in the field of marine biomaterials.