Rechargeable aqueous zinc anodes have gained tremendous attention because of their merits of intrinsic safety, low cost, and high theoretical volumetric capacity (5854 mAh cm-3 for Zn metal). In aqueous electrolytes, zinc anodes suffer from severe dendritic metal deposition. The regulation of Zn by inducing Zn-alloying metals has been reported. However, the underlying mechanisms have remained elusive. Here, for the first time, we did a comprehensive analysis to elucidate the mechanisms for the seeded and nondendritic growth of Zn on alloy anodes. We achieved uniform Zn deposition by introducing a Zn-alloying and soluble metal, Ag, on Zn anodes. Due to a shift of thermodynamic potential and the spatial confinement, the Ag-modified Zn anode exhibited improved overall cycling performance compared with previous deep-cycle Zn anodes. Furthermore, the seeded Zn deposition was visualized in operando for the first time using an optical microscope. The alloy-seeding design principle here can potentially be applied to improve the rechargeability of other metal anodes.