Abstract: Pt-based nanocatalysts offer excellent prospects for various industries. However, the low loading of Pt with excellent performance for efficient and stable nanocatalysts still presents a considerable challenge. In this study, nanocatalysts with ultralow Pt content, excellent performance, and carbon black as support were prepared through in-situ synthesis. These ~2-nm particles uniformly and stably dispersed on carbon black because of the strong s–p–d orbital hybridizations between carbon black and Pt, which suppressed the agglomeration of Pt ions. This unique structure is beneficial for the hydrogen evolution reaction. The catalysts exhibited remarkable catalytic activity for hydrogen evolution reaction, exhibiting a potential of 100 mV at 100 mA·cm⁻², which is comparable to those of commercial Pt/C catalysts. Mass activity (1.61 A/mg) was four times that of a commercial Pt/C catalyst (0.37 A/mg). The ultralow Pt loading (6.84wt%) paves the way for the development of next-generation electrocatalysts.