Seawater batteries (SWBs) have been mostly researched for large scale energy storage and (sub-)marine applications. In a SWB, the aqueous catholyte (seawater) and a non-aqueous anolyte (aprotic solvent solution) are physically separated by a NASICON solid electrolyte membrane. Given the practically unlimited Na⁺ ion supply from seawater, the energy storage is only limited by the amount of Na stored in the negative electrode. Therefore, the highest volumetric and gravimetric energy densities can be achieved by storing Na metal without the need for a host material. To achieve safe realization of such a cell, a compact, metal-less anode design is herein demonstrated for the first time. The anode compartment integrates the NASICON solid electrolyte, a Na-ion conductive solid-state polymer electrolyte (Na-SPE), having a high ionic conductivity (over 1 mS cm⁻¹ at moderate room temperature), and a negative electrode current collector. The reactive Na metal is not employed in the cell construction, but it is harvested from seawater upon charge (power to metal) and reconverted into energy upon the discharge process (metal to power). The overall round-trip energy efficiency (RTE) of the devices is over 85% at room temperature.