The battery safety and cost remain major challenges for developing next-generation rechargeable batteries. All-solid-state sodium (Na)-ion batteries are a promising option for low-cost as well as safe rechargeable batteries by using abundant resources and solid electrolytes. However, the operation of solid-state batteries is limited due to the low ionic conductivity of solid electrolytes. Therefore, it is essential to develop new compounds that feature a high ionic conductivity and chemical stability at room temperature. Herein, we report a potassium-substituted sodium superionic conductor solid electrolyte, Na3–xKxZr2Si2PO12 (0 ≤ x ≤ 0.2), that exhibits an ionic conductivity of 7.734 × 10–4 S/cm–1 at room temperature, which is more than 2 times higher than that of the undoped sample. The synchrotron powder diffraction patterns with Rietveld refinements revealed that the substitution of large K-ions resulted in an increased unit cell volume, widened the Na diffusion channel, and shortened the Na–Na distance. Our work demonstrates that substituting a larger cation on the Na site effectively widens the ion diffusion channel and consequently increases the bulk ionic conductivity. Our findings will contribute to improving the ionic conductivity of the solid electrolytes and further developing safe next-generation rechargeable batteries.