AbstractHydrazine‐assisted water electrolyzer is a promising energy‐efficient alternative to conventional water electrolyzer, offering an appealing path for sustainable hydrogen (H₂) production with reduced energy consumption. However, such electrolyzer is presently impeded by lacking an efficient catalyst to accelerate the kinetics of pivotal half‐reaction, that is, hydrazine oxidation reaction (HzOR). Herein, a ruthenium (Ru) single‐atom on an octahedral cobalt oxide (Co₃O₄) substrate (Ru‐Co₃O₄) catalyst, guided by theoretical calculations is developed. Those lattice‐confined Ru sites within octahedral structure of spinel Co₃O₄ effectively lower the energy barrier required for the formation of N₂H₂^* intermediate and desorption of H^* species in HzOR. As a result, the Ru‐Co₃O₄ catalyst achieves superior HzOR performance with a low potential of −0.024 V versus (vs.)reversible hydrogen electrode (RHE) at 100 mA cm⁻² and remarkable stability for over 200 h at 200 mA cm⁻². Importantly, a modular H₂ production achieves an output of 0.48 kWh electricity per m³ H₂ by decoupling and pairing the HzOR and hydrogen evolution reaction (HER) half‐reaction with a Zinc (Zn) redox reservoir. The work represents a significant advancement in the field, offering substantial flexibility for on‐demand H₂ production and energy output.