Increasing the active site density of single atom catalysts (SACs) is expected to generate closely neighboring atomic sites with potential synergetic interaction. However, the synthesis of SACs with high active site density still remains a great challenge due to the easy aggregation of high density metal atoms during the synthesis. In the present work, we develop a stepwise anchoring strategy for the large-scale preparation of carbon-supported high-density Pt SACs (denoted as PtSA@BP). The Pt loading of PtSA@BP is as high as 2.5 wt%, leading to the observation of abundant closely distanced single Pt sites. The produced PtSA@BP catalyst exhibits ultrahigh catalytic activity for the alkaline hydrogen evolution reaction with a low overpotential of 26 mV at 10 mA cm−2 in 1.0 M KOH under ultralow Pt loadings of 0.0009 mgPt cm−2 on the electrode, much superior to commercial Pt/C (20 wt%). Mechanistic studies suggest the main contribution of the coordination of closely distanced three-coordinated PtC2N1 moieties to the excellent catalytic activities towards the conversion of water to H2, due to their close-to-zero metal–hydrogen binding value and intense adsorption capability to H2O molecules as well as their low water-dissociation energy barrier. More importantly, this strategy has been verified to be feasible for preparing other noble-metal based SACs, for example, Rh and Pd. The present result provides an enabling and versatile platform for facile access of SACs with technological importance in various areas.