Abstract The overall performance of sodium-ion batteries, particularly regarding safety and cycle life, remains below expectations due to severe degradation of electrode materials and the electrode/electrolyte interphase. Herein, we develop a smart gel polymer electrolyte for hard carbon||NaNi1/3Fe1/3Mn1/3O2 batteries through the in situ radical polymerization of a cyanoethylurea-containing methacrylate monomer and an isocyanate-based methacrylate monomer in conventional NaPF6-carbonate-based electrolytes. We demonstrate that the smart gel polymer electrolyte facilitates the formation of robust electrode/electrolyte interphase layers, thus improving the thermal and chem-electrochemical stability of the electrodes. When the temperature exceeds 120 °C, the in situ formed gel polymer electrolyte undergoes further crosslinking through nucleophilic addition reactions between urea and isocyanate motifs. This additional crosslinking blocks ion transportation and inhibits crosstalk effects, thus boosting the safety of pouch-type hard carbon||NaNi1/3Fe1/3Mn1/3O2 batteries. Moreover, the smart gel polymer electrolyte enables hard carbon||NaNi1/3Fe1/3Mn1/3O2 full cells to achieve improved cycle life even at the elevated temperature of 50 °C. The design philosophy behind the development of in situ formed smart gel polymer electrolytes offers valuable guidance for creating high-safety, long-life, and sustainable sodium-ion batteries.