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Solid-state batteries with nonflammable inorganic solid electrolytes provide a fundamental solution for resolving safety concerns. Li7La3Zr2O12 (LLZO) has been considered as a promising candidate for solid electrolytes, due to its high Li+ conductivity and chemical/electrochemical compatibility with Li metal. However, LLZO electrolytes are known to react with H2O and CO2 to form lithium carbonates (Li2CO3) on the surface when exposed to the ambient air, resulting in the significant degradation in Li+-conduction properties. Herein, we propose an effective approach for improving the air-stability of LLZO via hydrophobic polymer encapsulation. For encapsulation of LLZO powders, polyurethane-based polymers are designed and synthesized to have high hydrophobicity and ionic conductivity by engineering soft segment and hydrophobic chain extender. Bare LLZO and polymer-encapsulated LLZO (P-LLZO) powders are subjected to accelerated durability tests (ADTs) in which the concentrations of O2, H2O, and CO2 are precisely controlled to promote the interfacial reactions. Surface characterization studies reveal that the polymer encapsulation of LLZO effectively mitigates the interfacial degradation (Li2CO3 formation) by preventing the direct contact between LLZO and H2O/CO2. Furthermore, a biphasic solid electrolyte (BSE) fabricated using ADT-tested P-LLZO powders exhibits higher ionic conductivity as compared with that of BSE with ADT-tested LLZO, proving the efficacy of the polymer encapsulation. The findings of this study would be essential in understanding the role of interfacial engineering in mitigating the degradation of Li+-conduction properties and developing highly conductive and stable LLZO solid electrolytes.