Adhesive composite electrolytes are of great interest due to the potential in achieving a good balance among electrochemical, mechanical and interfacial properties. However, in composite electrolytes, the fillers are usually rigid inorganic nanoparticles with limited functional groups contributing to mechanical properties and/or adhesion properties. In this study, abundant proteins are employed as unique additive for fabricating composite electrolytes with notable improvement in not only mechanical strength, but also adhesion properties. It is found that the introduced protein forms uniform nanoparticles in the final composite electrolyte. The influence of the protein nanofiller on the electrolyte properties (mechanical, ionic conductivity and adhesion properties) is investigated. The results indicate that proteins, particularly gelatin protein, can notably improve not only the mechanical properties (modulus and elasticity), but also the adhesion properties, while the high ionic conductivity of the electrolyte matrix is maintained. Benefited from the above significant properties of the composite electrolyte, the electrochemical performance of a half-cell with the gelatin-reinforced composite electrolyte is notably improved as compared with the cell with conventional liquid electrolyte. This study indicates that natural proteins can be a promising functional bio-filler for composite electrolytes. (C) 2016 Elsevier Ltd. All rights reserved.