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We report the use of phosphorus-doped carbon (P-C) as support to grow ultrasmall (1–3 nm) and ligand-free precious metal nanocrystals (PMNCs) via chemical reduction. We show that the valence states of surface phosphorus species are critical in tuning the affinity between the carbon support and metal precursors, which rationally controls the loading size and uniformity of resultant PMNCs. Five kinds of PMNCs, including Ru, Ag, Au, Rh, and Pd, were grown in situ to demonstrate the key role of surface phosphorus sites on the P-C support. As a proof-of-concept application, Ru nanocatalysts with an average diameter of 1.0±0.2 nm supported on P-C were examined for the electrocatalytic hydrogen evolution reaction (HER). Ultrasmall and ligand-free Ru nanocatalysts exhibited superior HER activity and stability compared to its counterparts with surface agents or larger sizes. An overpotential of 27.6 mV (vs. reversible hydrogen electrode) for Ru nanocatalysts was achieved at a current density of 10 mA cm−2. This novel method opens a new avenue to immobilize ligand-free and well-dispersed PMNCs on carbon; and, more importantly, it provides a new library of supported PMNCs with high catalytic activity. [ABSTRACT FROM AUTHOR]