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Electrocatalytic water splitting is widely regarded as a sustainable and promising technology for the production of hydrogen. However, their efficiency is severely limited by the sluggish kinetics of the oxygen evolution reaction. This work demonstrates that through the rational design of single-atom catalysts (SACs), hydrazine oxidation can function as an efficient alternative anodic reaction to replace the oxygen evolution reaction, thereby substantially lowering the energy input required for hydrogen production. Using density functional theory calculations, we systematically investigated a series of single-atom 3d transition metals ranging from Sc to Zn anchored on four types of defective graphene substrates. Among the investigated systems, Cr@N4 and Fe@N4 stand out as thermodynamically promising bifunctional catalysts, exhibiting low reaction free energy barriers of 0.33 and 0.48 eV, respectively, together with high thermodynamic stability and selectivity. Meanwhile, both catalysts also demonstrate excellent hydrogen evolution reaction activity, with near-optimal hydrogen adsorption free energies of 0.09 eV for Cr@N4 and 0.11 eV for Fe@N4. Electronic structure analyses reveal a pronounced electron–acceptance–donation interaction between the single-atom active centers and N2H4 molecules, which enhances adsorption and facilitates the reaction process.