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To improve the energy density of quasi‐solid‐state polymer electrolyte (QPE)‐based supercapacitors, the design of zwitterion‐containing QPE (ZQPE) with a high dielectric constant (ɛs) is proposed. This enhancement would increase the capacitance of ZQPE‐based supercapacitors, given the direct proportionality between capacitance and dielectric constant. The optimized ZQPE exhibits a high ionic conductivity (2.2 × 10−3 S cm−1) and good mechanical robustness (4.1 × 104 Pa) at room temperature. The ZQPE shows a much higher static dielectric constant (ɛs = 69) than the zwitterion‐free QPE (ɛs = 48), as supported by density functional theory calculations and molecular dynamics simulations. The flexible all‐solid‐state supercapacitor, fabricated with ZQPE and activated carbon electrodes, exhibits excellent electrochemical performance. The incorporation of zwitterion to QPE results in an increase in specific capacitance from 178 to 225 F g−1 with a wide operating potential window (0–4 V). The ZQPE‐based supercapacitor also exhibits a large energy density of 142 Wh kg−1 at a power density of 464 W kg−1 and remarkable long‐term electrochemical and mechanical cycling stability, retaining 80% of its capacitance retention over 10 000 cycles and even under rolling deformation for 1500 cycles. This strategy offers new insights for developing QPE designs for high energy density all‐solid‐state supercapacitors. [ABSTRACT FROM AUTHOR]