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Abstract This study presents a novel polymer‐in‐salt (PIS) zwitterionic polyurethane‐based solid polymer electrolyte (zPU‐SPE) that offers high ionic conductivity, strong interaction with electrodes, and excellent mechanical and electrochemical stabilities, making it promising for high‐performance all solid‐state lithium batteries (ASSLBs). The zPU‐SPE exhibits remarkable lithium‐ion (Li⁺) conductivity (3.7 × 10⁻⁴ S cm⁻¹at 25 °C), enabled by exceptionally high salt loading of up to 90 wt.% (12.6 molar ratio of Li salt to polymer unit) without phase separation. It addresses the limitations of conventional SPEs by combining high ionic conductivity with a Li⁺transference number of 0.44, achieved through the incorporation of zwitterionic groups that enhance ion dissociation and transport. The high surface energy (338.4 J m⁻²) and elasticity ensure excellent adhesion to Li anodes, reducing interfacial resistance and ensuring uniform Li⁺flux. When tested in Li||zPU||LiFePO₄ and Li||zPU||S/C cells, the zPU‐SPE demonstrated remarkable cycling stability, retaining 76% capacity after 2000 cycles with the LiFePO₄cathode, and achieving 84% capacity retention after 300 cycles with the S/C cathode. Molecular simulations and a range of experimental characterizations confirm the superior structural organization of the zPU matrix, contributing to its outstanding electrochemical performance. The findings strongly suggest that zPU‐SPE is a promising candidate for next‐generation ASSLBs.