Hydrogenation reactions are industrially important reactions that typically require unfavorably high H2 pressure and temperature for many functional groups. Herein we reveal surprisingly strong size‐dependent activity of Pt nanoparticles (PtNPs) in catalyzing this reaction. Based on unambiguous spectral analyses, the size effect has been rationalized by the size‐dependent d‐band electron structure of the PtNPs. This understanding enables production of a catalyst with size of 1.2 nm, which shows a sixfold increase in turnover frequency and 28‐fold increase in mass activity in the regioselective hydrogenation of quinoline, compared with PtNPs of 5.3 nm, allowing the reaction to proceed under ambient conditions with unprecedentedly high reaction rates. The size effect and the synthesis strategy developed herein may provide a general methodology in the design of metal‐nanoparticle‐based catalysts for a broad range of organic syntheses. Cat. size: The activity of Pt nanoparticles (PtNPs) in catalyzing the regioselective hydrogenation of quinoline (see picture) is strongly dependent on the size of the PtNPs. This dependence is rationalized by the size‐dependent d‐band electron structure of the PtNPs and thus their interactions with the reactants and the products.