The solid-state electrolyte in a solid-state battery acts as an electrons' barrier and an ions' bridge between the two electrodes. As solid-state electrolyte does not store the mobile ions, it is necessary to achieve a thin solid-state electrolyte to reduce the internal resistance and enhance the energy density. In this work, a thin NASICON solid-state electrolyte, with a stoichiometry of Na₃Zr₂Si₂PO₁₂, is fabricated by the tape-casting method and its thickness can be easily controlled by the gap between substrate and scraper. The areal-specific resistance and the flexural strength increase with the electrolyte thickness. A solid-state sodium metal battery with 86 μm thick Na₃Zr₂Si₂PO₁₂ exhibits a reversible specific capacity of 73–78 mAh g⁻¹ with a redox potential of 3.4 V at 0.2 C. This work presents the importance of electrolyte thickness to reduce internal resistance and achieve a high energy density for sodium batteries.