Seawater batteries (SWBs) have emerged as a next-generation battery technology that does not rely on lithium, a limited resource essential for lithium-ion batteries. Instead, SWBs utilize abundant sodium from seawater, offering a sustainable alternative to conventional battery technologies. Previous studies have demonstrated the feasibility of achieving high energy densities in SWB anodes using vertically aligned electrodes. However, the use of tin anode materials with high volumetric energy density has encountered reversibility challenges due to the electrical isolation of tin particles caused by severe pulverization during charging and discharging. In this study, the reversibility of vertically arranged tin electrodes is improved by promoting desodiation of pulverized tin particles through the use of sodium-pyrene (Na-Pyr) as a redox mediator. The Na-Pyr redox-mediated electrolyte, combined with vertically aligned tin electrodes, demonstrates reversible capacities of 6 mAh cm−2 over 80 cycles in SWBs. Furthermore, it is shown that arranging the electrodes vertically to maximize the area can achieve a high areal capacity of up to 40 mAh cm−2. The combination of the Na-Pyr redox mediator and vertical tin electrode, with its excellent electrochemical performance, is promising as a practical anode material for enabling SWBs to achieve high energy density.