Sodium alginate, a natural anionic polysaccharide, exhibits broad potential applications in food, biomedicine, and environmental engineering due to its favorable biocompatibility, degradability, and functional tunability. This review systematically summarizes its chemical structure, physicochemical characteristics, sources, and extraction methods. It also focused on modification strategies, including chemical approaches (e.g., esterification, oxidation, sulfation, graft copolymerization), physical methods (composite modification, irradiation cross-linking, ultrasound treatment), and biological (e.g., enzyme regulation), and elucidated their underlying mechanisms. In the context of food science, special emphasis is placed on food-compatible chemistries and mild modification routes (such as phenolic crosslinking, enzyme-assisted coupling, and other green reactions) that enable the development of edible films, coatings, and functional carriers, while distinguishing these from non-food-oriented chemical strategies. The review further highlights novel applications of modified sodium alginate in areas including food packaging, functional delivery systems, drug release, tissue engineering, and environmental remediation (heavy metal and dye removal). Overall, this work provides a comprehensive perspective linking modification pathways to food-relevant applications and clarifies how chemical tailoring of alginate contributes to the design of safe, sustainable, and high-performance bio-based materials.