“Single‐phase” all‐solid‐state battery, based on Na3−xV2−xZrx(PO4)3 as the cathode, anode, and electrolyte, is reported. It successfully undergoes room temperature charge–discharge reactions based on the V3+/V2+ (anode) and V3+/V4+ (cathode) redox systems with insertion reaction. The interfacial resistance is essentially zero at the anode and cathode.