AbstractRechargeable seawater batteries have gained recognition as key sustainable electrochemical systems by employing the near‐infinite and eco‐friendly catholyte seawater. However, their practical applications have been limited owing to the low chemical and electrochemical stability of the anode component. Herein, a stability‐secured approach was developed by using sodium‐biphenyl‐dimethoxyethane solution as a redox‐active functional anolyte for high‐performance seawater batteries. This anolyte system shows high electrochemical stability, superior cycle performance, and cost‐effectiveness over conventional electrolyte systems.