Polymer electrolytes with robust mechanical properties and high ionic conductivity are highly desirable for solid-state energy storage devices. Therefore, in this study, a polymer electrolyte based on a zwitterionic graft copolymer, poly(vinyl alcohol)-graft-poly(sulfobetaine methacrylate) (PVA-g-PSBMA), is developed. Herein, we designed a graft copolymer that utilizes the excellent film-forming nature of PVA main chains and ion-conductive properties of polyzwitterionic PSBMA side chains. The PVA-g-PSBMA electrolytes exhibited microphase separated structures, wherein H₃PO₄ interacted more strongly with the PSBMA domains. Therefore, the transport of proton ions was facilitated through the well-defined PSBMA-based ion channels, leading to a significant increase in the ionic conductivity. The polymer electrolyte was applied to the electric double-layer capacitor (EDLC) with activated carbon-based electrode. In comparison with the neat conventional PVA-based cell, the solid EDLC with PVA-g-PSBMA zwitterionic graft copolymer electrolytes showed improved specific capacitance (46.6 F g^-1), energy density (3.67 Wh kg^-1), and power density (186 W kg^-1). The improved performance can be attributed to the higher ionic conductivity and better interfacial compatibility of electrode/electrolyte for the PVA-g-PSBMA electrolytes.