Aqueous zinc-ion batteries (ZIBs) gain attention as promising energy storage devices due to their high safety. However, the narrow electrochemical window and unfavorable side reactions induced by water decomposition restrict their development. Thus, confining water activity to enhance stability and enlarge the electrochemical window is required. Herein, a 2.9 m (mol kgsolvent−1) Zn(ClO4)2−CO(NH2)2−H2O ternary aqueous eutectic electrolyte is prepared with restricted water activity at room temperature. The strong intermolecular interactions between CO(NH2)2 and H2O decrease the free H2O molecules and reduce their activity to suppress the parasitic reactions. Compared to conventional aqueous electrolytes, this urea-modified electrolyte exhibits similar ionic conductivity (6.83 mS cm−1) and viscosity (29.5 mPa s) but with a significantly expanded electrochemical stability window (2.6 V) than the conventional one (1.7 V). Additionally, the preferential adsorption and reduction of urea molecules on the zinc surface mediate the formation of an organic solid electrolyte interphase, which passivates the anode and facilitates homogeneous zinc deposition. As a result, this ternary aqueous electrolyte enables high-voltage zinc/vanadium batteries with a capacity of 125 mAh g−1 for 300 cycles at 5 A g−1. This finding demonstrates a low-cost and practicable approach for realizing stable aqueous zinc-ion batteries with an enlarged electrochemical stability window.