Electrocatalytic hydrogen evolution reaction (HER) is widely regarded as one of the most efficient and sustainable strategies for hydrogen production. Up to now, most electrocatalysis research related to HER mainly focuses on stand-alone electrocatalysis and fails to pay attention to the integration of other driving forces such as light. Herein, Cu2 O nanostructures with different exposed crystal facets were synthesized by wet chemical methods for electrocatalytic HER, and it was found that the octahedral Cu2 O nanostructures with exposed crystal planes of (111) (O-Cu2 O) had the best hydrogen evolution performance. Density functional theory (DFT) calculations found that the better HER performance on Cu2 O (111) facets was attributed to the lower energy barrier in the Heyrovsky step. Operando Raman spectroscopy and ex-situ characterization techniques showed that Cu2 O was reduced during HER, in which Cu dendrites were grown on the surface of the Cu2 O nanostructures, resulting in the better HER performance of O-Cu2 O after HER (O-Cu2 O-A) compared with that of the as-prepared O-Cu2 O. DFT calculations indicated that the charge transfer at the Cu2 O/Cu interface enhanced its surface electron concentration. Under illumination, the onset potential of O-Cu2 O-A is ca. 52 mV positive than that of O-Cu2 O, which is induced by the plasmon-activated electrochemical system consisting of Cu2 O and the in-situ generated Cu dendrites. Incident photon-to-current efficiency (IPCE) measurements, ultraviolet-visible (UV-Vis) spectroscopy and X-ray photoelectron spectroscopy (XPS) demonstrate the hot electron injection (HEI) from Cu dendrites to Cu2 O. Ab initio nonadiabatic molecular dynamics (NAMD) simulations revealed that the transfer of photogenerated electrons (27 fs) from Cu dendrites to Cu2 O nanostructures is faster than electron relaxation (170 fs), enhancing its surface plasmons activity, and the HEI of Cu dendrites increases the charge density of Cu2 O. These make the energy level of the catalyst be closer to that of H+ /H2 , evidenced by the plasmon-enhanced HER electrocatalytic activity. This article is protected by copyright. All rights reserved.