Abstract The development of efficient and stable catalysts for hydrogen production from electrolytic water for multiscenario applications is of great significance to alleviate the energy crisis. Here, highly dispersed PtRu alloy cluster electrocatalysts (PtRu/NCNTs) with ultrasmall particle sizes are constructed by simultaneous in situ reduction method with the help of in situ reduction properties of borane clusters. PtRu/NCNTs achieves significant hydrogen production from electrolytic water in multiple scenarios (full pH environment, real seawater, simulated seawater, and anion‐exchange membrane integration), exceeding the performance of commercial Pt/C catalysts. The interfacial adsorption of H 2 O and the transition behavior of [H] on the Pt and Ru sites in the hydrogen evolution reaction (HER) are observed by in situ attenuated total reflection fourier transform infrared spectroscopy, and density‐functional theory (DFT) calculations revealed that electron transfer from Ru to Pt in the PtRu/NCNTs yielded closer‐to‐zero Gibbs free energies and smaller water‐cleavage energies to enhance the HER activity.