The ultimate goal of catalyst design is to optimize catalytic performance by revealing the relationship between structure and activity. However, it remainstremendously challenging to distinguish the role of electronic and ensemble effects in bimetallic core-shell electrocatalysts for hydrogen evolution reaction (HER). For this purpose, we demonstrate an efficient approach to local epitaxial growth of Rh atoms on Au decahedra to investigate the above-mentioned two effects. Among them, Au75Rh25 core-shell star-shaped decahedra exhibited the best activity for HER with anoverpotential of 64.1 mV at a current density of 10 mA cm−2, which is about 39.4 mV less than that of commercial Rh/C. In addition, it is noteworthy that Tafel slope is also comparable (33.8 mV dec–1) with Pt/C (30.2 mV dec–1). Density function theory (DFT) calculations revealed that ensemble effect is the dominant factor to accelerate the reaction kinetics of HER byexposing Au atoms on the surfaceand thus weakening the strong adsorption of the intermediates on Rh surface.