Solid‐state sodium‐metal batteries (SSBs) hold great promise for their merits in low‐cost, high energy density, and safety. However, developing solid electrolyte (SE) materials for SSBs with high performance is still a great challenge. In this study, high‐entropy Na4.9Sm0.3Y0.2Gd0.2La0.1Al0.1Zr0.1Si4O12 was synthesized at comparatively low sintering temperature of 950 °C with high room‐temperature ionic conductivity of 6.7×10−4 S cm−1 and a low activation energy of 0.22 eV. More importantly, the Na symmetric cells using high‐entropy SE show a high critical current density of 0.6 mA cm−2, outstanding rate performance with fairly flat potential profiles at 0.5 mA cm−2 and steady cycling over 700 h under 0.1 mA cm−2. Solid‐state Na3V2(PO4)3||high‐entropy SE||Na batteries are further assembled manifesting a desirable cycling stability with almost no capacity decay after 600 cycles and high Columbic efficiency over 99.9 %. The findings present opportunities for the design of high‐entropy Na‐ion conductors toward the development of SSBs. [ABSTRACT FROM AUTHOR]