Hydrazine-assisted water electrolysis is a promising energy-saving alternative to conventional water splitting; however, noble-metal catalysts suffer severe deactivation from nitrogen-intermediate poisoning during sustained anodic operation. Herein, amorphized and oxidized layered PtSe2 is presented as an all-in-one bifunctional electrocatalytic platform solving this challenge. Plasma treatment induces simultaneous surface oxidation, amorphization, and Pt clusterization to form vertically integrated Pt–PtSexOy heterostructures, as verified by transmission electron microscopy and x-ray photoelectron spectroscopy. Density functional theory and in situ Raman spectroscopy reveal a potential-dependent reaction network: N─H dehydrogenation dominates at low bias, while N─N cleavage becomes accessible at elevated potentials. Within this network, the amorphous PtSexOy matrix acts as an efficient electron sink and proton-accepting environment. By rendering Pt clusters optimally electron-deficient, it cooperatively facilitates proton-coupled electron transfer during N─H activation and fundamentally suppresses the accumulation of strongly bound nitrogen intermediates, resulting in enhanced intrinsic hydrazine oxidation activity with increased turnover frequencies. Simultaneously, the formation of Pt clusters within this modulated coordination environment drastically boosts the cathodic hydrogen evolution reaction beyond commercial Pt/C benchmarks. Leveraging this synergistic bifunctionality, a symmetric two-electrode electrolyzer achieves highly energy-efficient overall water splitting, delivering long-term stability and demonstrating state-of-the-art performance among transition metal compound-based electrocatalysts.