Modulation of the electronic structure of metal catalysts is an effective approach to optimize the electrocatalytic activity. In this study, we discover surprisingly strong activation effects of black phosphorus (BP) on platinum (Pt) catalysts. In this way, the surface electronic structure of Pt can be effectively and rapidly modulated to produce greatly enhanced catalytic activity in the hydrogen evolution reaction (HER). The unique and negative binding energy between BP and Pt leads to spontaneous formation of Pt-P bonds producing strong synergistic ligand effects on the Pt nanoparticles, but no Pt-P bond can be formed on red phosphorus which is another allotrope of P. Theoretical and experimental assessments demonstrate that the Pt-P bonds alter the intrinsic electronic structure and hydrogen adsorption free energy of Pt. Furthermore, by controlling the amount of Pt-P bonds, 3.5-folds enhancement in the HER activity can be achieved from the BP-activated Pt catalyst and the activity is 6.1 times higher than that of the state-of-the-art commercial Pt/C catalyst. The BP-activated Pt catalyst exhibits a current density of 82.89 mA cm-2 with only 1 μg of Pt in 1 M KOH at an overpotential of 70 mV and the characteristics are superior to those of state-of-the-art catalysts in the literature. This work reveals a practical strategy to manipulate the electronic structure of Pt catalysts by exploiting the unique activation effects of BP and provides a new strategy for the synthesis of highly efficient Pt-based catalysts for diverse catalytic reactions and applications.