Seafood by-products constitute 30–85% of the total fish weight and are often discarded despite containing nutrients and bioactive components. Fish skin provides collagen and gelatin; fish bone contains collagen, gelatin, and minerals like calcium phosphate and hydroxyapatite; and fish viscera offer proteases with high catalytic activity. Marine oils are rich in polyunsaturated fatty acids (PUFAs), especially eicosapentaenoic acid and docosahexaenoic acid, and fat-soluble vitamins. Shellfish wastes contain calcium, protein, chitin, lipids, and carotenoids, while macroalgae are rich in sulfated polysaccharides, phlorotannins, PUFAs, pigments, vitamins, and minerals. Marine collagen is used in biocompatible glues, drug carriers, and bone/cartilage formation. Collagen hydrolysates are common in skin care and food preservation, and marine gelatin is used as a clarifying, emulsifying, stabilizing, and low-caloric agent. Marine proteases are employed in food fermentation, flavor enhancement, and as detergents. Seafood, shellfish, and algae are primary sources of omega-3 PUFAs with numerous health benefits, used in supplements and cosmetics. Chitosan and derivatives, from chitin of crustacean’s exoskeleton, are used in tissue engineering, wound treatment, and cosmetics. Macroalgae-derived sulfated polysaccharides are used as stabilizing and emulsifying agents in food, while carotenoids like astaxanthin are employed in coloring, food packaging, and cosmetic formulations. Hydroxyapatite from fish bone is valuable in dentistry and orthopedics for bone regenerator due to its high biocompatibility and no immunogenicity. Thus, seafood by-products, including shellfish and macroalgae, are excellent sources of bioactive components with numerous applications in biomedical, food, and cosmetic industries, adding a significant value to these residues.