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Matrix-supported metal nanoclusters (1–10 nm) with unique size- and shape-dependent properties have drawn attention for their potential applications in electronics, catalysis, energy storage, and sensors. However, synthesis of matrix-supported ultrasmall nanoclusters at high concentration and in an unaggregated state is challenging. Here we demonstrate a rapid laser pulse technique to in situ fabricate ultrasmall metal nanoclusters supported on a carbon nanofiber (CNF) matrix with kinetically controllable size and surface density. A rapid laser pulse heating on the metal precursor incorporated CNF matrix triggers the fast nucleation and growth of metal nanoclusters, and a subsequent ultrafast quenching freezes them onto the CNF structure. We find that a shorter laser pulse enables the formation of metal nanoclusters with higher number densities and smaller sizes while longer laser pulse leads to the further growth of metal nanoclusters and the achievement of their equilibrium shape. A characteristic time analysis suggests that the growth of metal nanoclusters is dominated by surface diffusion and sintering, and Ostwald ripening is mainly involved at the early stage of nanocluster formation. We also demonstrate that the catalytic performance of CNF matrix supported metal nanoclusters toward electrocatalytic hydrogen evolution is enhanced for metal nanoclusters with a smaller size and higher number density. This work provides a promising approach for rapid and scalable fabrication of ultrasmall, high-density metal nanoclusters, and nanocluster-based devices. [ABSTRACT FROM AUTHOR]