The development of sufficiently effective catalysts with extremely superior performance for electrocatalytic hydrogen production still remains a formidable challenge, especially in acidic media. Here, we report ultrasmall high-entropy alloy (us-HEA) nanoparticles (NPs) with the best-level performance for hydrogen evolution reaction (HER). The us-HEA (NiCoFePtRh) NPs show an average diameter of 1.68 nm, which is the smallest size in the reported HEAs. The atomic structure, coordinational structure, and electronic structure of the us-HEAs were comprehensively clarified. The us-HEA/C achieves an ultrahigh mass activity of 28.3 A mg-1noble metals at -0.05 V (vs the reversible hydrogen electrode, RHE) for HER in 0.5 M H2SO4 solution, which is 40.4 and 74.5 times higher than those of the commercial Pt/C and Rh/C catalysts, respectively. Moreover, the us-HEA/C demonstrates an ultrahigh turnover frequency of 30.1 s-1 at 50 mV overpotential (41.8 times higher than that of the Pt/C catalyst) and excellent stability with no decay after 10 000 cycles. Operando X-ray absorption spectroscopy and theoretical calculations reveal the actual active sites, tunable electronic structures, and a synergistic effect among five elements, which endow significantly enhanced HER activity. This work not only engineers a general and scalable strategy for synthesizing us-HEA NPs and elucidates the complex structural information and catalytic mechanisms of multielement HEA system in depth, but also highlights HEAs as sufficiently advanced catalysts and accelerates the research of HEAs in energy-related applications.