<p>Seaweed, an abundant third-generation biomass, has garnered significant interest for hydrocolloid extraction due to its unique composition. The extraction of these hydrocolloids, coupled with the negligible presence of lignin, leaves behind a cellulose-rich residue ideal for nanocellulose production—a potential that remains underexplored. Nanocellulose, known for its versatility, finds applications across composite, fibre, and medical industries. This review delves into the structure, composition, and extraction processes of seaweed hydrocolloids. It also investigates green emerging pretreatment techniques for hydrocolloid extraction, evaluating their advantages and limitations. This review further analyses nanocellulose produced from seaweed residues, focusing on treatment types, morphology, thermal stability, and crystallinity to determine optimal applications. Lastly, a comprehensive biorefinery approach is proposed, integrating hydrocolloid extraction and nanocellulose production to maximize the benefits from the seaweed industry.</p>