AbstractNASICON‐structured solid‐state electrolytes (SSEs) are highly promising materials for sodium solid‐state metal batteries (NaSMBs). However, the current synthesis methods are often labor‐intensive and inefficient, consuming a significant amount of energy and time. Here, an ultrafast high‐temperature synthesis (UHS) technique is successfully demonstrated to directly synthesize NASICON‐type SSEs from mixed precursor powders, reducing the synthesis time from hours to merely seconds. The intermediate with a Na₃PO₄ structure plays a critical role in the rapid synthesis of NASICON‐type SSEs, ultimately leading to the formation of the final NASICON phase. Moreover, the UHS‐synthesizes NASICON‐type Na_3.3Zr_1.7Lu_0.3Si₂PO₁₂ (NZLSP) exhibits high room temperature ionic conductivity of 7.7 × 10⁻⁴ S cm⁻¹, approximately three times that of the undoped Na₃Zr₂Si₂PO₁₂ (NZSP). The Na|NZLSP|Na symmetric cell can sustain highly stable cycling for over 4800 h. This study provides a novel insight and validation in the precise and targeted synthesis of complex oxide solid‐state electrolytes.