This review discusses recent advances in biopolymer-based hydrogel electrolytes for flexible supercapacitors. Biopolymers, such as polysaccharides (e.g., cellulose, alginate, chitin/chitosan) and protein-based polymers (e.g., silk, gelatin), are attractive due to their natural abundance, low cost, biodegradability, good biocompatibility, and sustainability. They offer tunable morphology and mechanical properties, along with abundant reactive sites for chemical modification, making them appealing building blocks for sustainable electronic devices. Their functional groups enable efficient ion migration, and nanocellulose materials exhibit high mechanical strength, structural flexibility, and tunable self-assembly behavior. These properties make biopolymers suitable for fabricating multifunctional gel polymer electrolytes (GPEs). Biopolymer-based GPEs have been used extensively in supercapacitor applications. Cellulose is the most abundant natural biopolymer, with a high degree of crystallinity and strong hydrogen-bond networks, providing high stability and axial stiffness. Chitosan, gelatin, and alginate are also used as biopolymer-based GPEs. These materials can be used as matrices or additives to develop diverse hydrogels, ionogels, and organogels, which find utility in various types of supercapacitors. The electrolyte serves as the ionic conductor between the two electrodes in supercapacitors, significantly influencing electrochemical performance. Gel polymer electrolytes (GPEs), which contain immobilized liquid electrolytes in a polymer matrix, have been proposed to act as both separator and electrolyte, reducing the risk of leakage and evaporation observed in devices using liquid electrolytes and the rigidity of solid electrolytes. Biopolymers offer various advantages, including dielectric and piezoelectric properties, abundant hydroxyl groups for functionalization, and rich inter- and intramolecular hydrogen bonds for mechanical properties. They can also be used to form entangled network structures with controlled porous microstructures, making them suitable for separators or gel electrolytes to facilitate ionic transportation. The review discusses the structure and properties of typical biopolymers and their roles in electrolytes. It outlines the energy storage mechanisms of supercapacitors, revealing the pivotal roles of biopolymers in facilitating ion migration within GPEs. The performance evaluation standards of biopolymer-based GPEs for flexible supercapacitors are highlighted, along with various strategies to enhance their performance, such as optimizing interface contact and voltage windows. The review also discusses the mechanical properties of biopolymer-based GPEs, emphasizing their importance in enabling mechanical flexibility to withstand diverse types of mechanical deformations without compromising electrochemical performance. Cross-linking strategies, such as physical and chemical cross-links, are discussed as methods to enhance the mechanical properties of biopolymer-based GPEs. The review highlights the self-heThis review discusses recent advances in biopolymer-based hydrogel electrolytes for flexible supercapacitors. Biopolymers, such as polysaccharides (e.g., cellulose, alginate, chitin/chitosan) and protein-based polymers (e.g., silk, gelatin), are attractive due to their natural abundance, low cost, biodegradability, good biocompatibility, and sustainability. They offer tunable morphology and mechanical properties, along with abundant reactive sites for chemical modification, making them appealing building blocks for sustainable electronic devices. Their functional groups enable efficient ion migration, and nanocellulose materials exhibit high mechanical strength, structural flexibility, and tunable self-assembly behavior. These properties make biopolymers suitable for fabricating multifunctional gel polymer electrolytes (GPEs). Biopolymer-based GPEs have been used extensively in supercapacitor applications. Cellulose is the most abundant natural biopolymer, with a high degree of crystallinity and strong hydrogen-bond networks, providing high stability and axial stiffness. Chitosan, gelatin, and alginate are also used as biopolymer-based GPEs. These materials can be used as matrices or additives to develop diverse hydrogels, ionogels, and organogels, which find utility in various types of supercapacitors. The electrolyte serves as the ionic conductor between the two electrodes in supercapacitors, significantly influencing electrochemical performance. Gel polymer electrolytes (GPEs), which contain immobilized liquid electrolytes in a polymer matrix, have been proposed to act as both separator and electrolyte, reducing the risk of leakage and evaporation observed in devices using liquid electrolytes and the rigidity of solid electrolytes. Biopolymers offer various advantages, including dielectric and piezoelectric properties, abundant hydroxyl groups for functionalization, and rich inter- and intramolecular hydrogen bonds for mechanical properties. They can also be used to form entangled network structures with controlled porous microstructures, making them suitable for separators or gel electrolytes to facilitate ionic transportation. The review discusses the structure and properties of typical biopolymers and their roles in electrolytes. It outlines the energy storage mechanisms of supercapacitors, revealing the pivotal roles of biopolymers in facilitating ion migration within GPEs. The performance evaluation standards of biopolymer-based GPEs for flexible supercapacitors are highlighted, along with various strategies to enhance their performance, such as optimizing interface contact and voltage windows. The review also discusses the mechanical properties of biopolymer-based GPEs, emphasizing their importance in enabling mechanical flexibility to withstand diverse types of mechanical deformations without compromising electrochemical performance. Cross-linking strategies, such as physical and chemical cross-links, are discussed as methods to enhance the mechanical properties of biopolymer-based GPEs. The review highlights the self-he