Developing rhodium-based electrocatalysts with strong metal-support interaction (SMSI) is crucial for hydrogen evolution reaction (HER) in full pH range, but it is still challenging. In this work, a boron doping strategy is proposed to activate the SMSI of ultra-small Rh nanoparticles encapsulated by N-doped carbon matrix (B-Rh@NC). Density functional theory (DFT) reveals that boron doping can regulate the electronic structure and the water adsorption energy, further boosting the HER activity. Consequently, the as-prepared B-Rh@NC nanospheres possess Pt-like activity for HER with low overpotentials in 1.0 M KOH (η10 = 26 mV), 0.5 M H2SO4 (η10 = 43 mV), and 1.0 M PBS (η10 = 70 mV), as well as outstanding stability. This work opens up a new way to construct remarkable pH-universal electrocatalysts.