An ultra-fine (∼1.9 nm) tri-metallic nanocatalyst (NC) comprised of a high density of PtRh nanoalloy over an NiOx support was developed with a Pt-loading of only 1.1 wt%. This material (denoted as PtRhNi) exhibited an outstanding HER performance in both acidic (0.5 M H2SO4) and alkaline (1.0 M KOH) media, outperforming the pure Pt catalyst with an exceptional mass activity (MA) of 4489 A gPt-1 and 1663 A gPt-1, respectively. Notably, PtRhNi NC achieved a remarkably lower overpotential (η) of 28 mV (at the cathodic current density of 10 mA cm-2) and Tafel slope of 30 mV dec-1 in acidic medium, while a similar overpotential (28 mV @ 10 mA cm-2) and competitive Tafel slope of 73 mV dec-1 were maintained in alkaline medium. More interestingly, this material retained a high performance in the chronoamperometric (CA) stability test for up to 6 h under a pH-universal environment. The experimental observations suggest that its enhanced HER performance originates from the synergistic effect between the neighbouring reaction sites in the sub-nanometer domains of PtRh nanoalloy, where the Rh-modifier and Pt-dimer sites synergistically trigger the proton adsorption (i.e., the discharge step) and reduction (i.e., the coupling of protons and electrons for accelerating the desorption step) kinetics, respectively. Besides, the RhOx and NiOx sites favour HO-H bond cleavage, and hence provide active centres for the dissociation of water. We envision that the proposed scenarios will open further opportunities for the rational design of Pt-based NCs with tailored structural and functional properties at the atomic level.