In this study, a novel mercapto-terminated trinuclear Ni(II) complex (Ni₃) was synthesized and used as an electrocatalyst for the detection of hydrazine hydrate in real water samples. The as-prepared Ni₃ molecule possesses six thiomethyl groups at its periphery and these SCH₃ groups can react with Au electrodes to immobilize the Ni₃ molecules on their surface through the formation of a self-assembled monolayer. The Ni₃-modified Au electrode (Ni₃/Au) demonstrates excellent electrocatalytic activity for the oxidation of hydrazine hydrate through a significant decrease in overpotential. The chronoamperometry study shows a diffusion coefficient (D) of 5.82 × 10⁻⁵ cm² s⁻¹ and a catalytic rate constant of 8.57 × 10³ M⁻¹ s⁻¹. Using the square wave voltammetry (SWV) technique, this Ni₃/Au electrode based hydrazine hydrate sensor exhibits a high sensitivity in quantitative analysis, and its detection limit could be as low as ∼0.07 μM with linearity ranging from 0.2 to 50 μM. In addition, due its good reproducibility, anti-interference performance, and long-term stability, the proposed sensor is capable of detecting trace levels of hydrazine hydrate in real water samples.