Electrocatalytic water splitting has been widely accepted as an environmentally friendly approach to generate clean H2. However, the sluggish oxidation half reaction, namely, the oxygen evolution reaction (OER), usually requires a high overpotential, which is the obstacle to high-efficiency overall water splitting to generate H2. Herein, we report the preparation of ultrathin nanosheet array Ni3S2 with a 9–14 nm nanosheet thickness grown directly on a commercial Ni foam substrate (Ni3S2/NF) by in situ vapor-phase hydrothermal (VPH) transformation of the nanosheet array structure Ni(OH)2/NF precursor. As a multifunctional electrocatalyst, the as-fabricated Ni3S2/NF-2 (VPH time of 2 h) displays excellent electrocatalytic activities toward the hydrazine oxidation reaction (HzOR) with a potential of 0.415 V (vs. RHE) to deliver a current density of 100 mA cm−2, an OER with an overpotential of 425 mV obtaining the same current density, and a hydrogen evolution reaction (HER) with an onset potential of −0.05 V (vs. RHE) in 1.0 M KOH media. A two-electrode system is therefore constructed using Ni3S2/NF-2 as both the anode and cathode, capable of achieving 100 mA cm−2 at 0.867 V in 1.0 M KOH with 0.2 M hydrazine. Density functional theory (DFT) calculations reveal that the adsorption of N2H4 molecules on the Ni3S2 (110) is more thermodynamically favourable than H2O, thus contributing to the high HzOR activity.