Rechargeable 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.