Herein, PtRu solid-solution alloy nanoparticles enable high-performance H-2 evolution reaction cocatalysts through the transfer of knowledge from electrocatalysis to photocatalysis. Ultrafine PtRu alloy nanoparticles (1.0 +/- 0.2 nm in diameter) are synthesized by using a chemical polyol method and used for electro/photo-catalyses. Integration of PtRu nanoparticles with carbon and Al-doped SrTiO3 powders via liquid-phase adsorption and successive two-step air and H-2 annealing ensures uniform deposition of PtRu alloy nanoparticles (<2 nm) with a solid-solution structure. The PtRu alloy nanoparticles exhibit higher electrocatalytic activity for the hydrogen evolution reaction than monometallic Pt counterparts. Modifying Al-doped SrTiO3 with PtRu alloy nanoparticles results in efficient photocatalytic overall water-splitting activity with an apparent quantum yield of 65% at 365 nm and a solar-to-hydrogen energy conversion efficiency of 0.45%, which are the highest values among SrTiO3-based photocatalytic water splitting systems with Rh-free cocatalysts. Transient diffuse reflectance spectroscopy reveals that the PtRu nanoparticles have a superior electron-capturing ability than the Pt counterparts, probably due to the decreased work function of Pt by alloying with Ru. This work establishes a framework to elucidate catalytic trends and provides rational design guidance toward the development of bimetallic solid-solution nanoparticles as cost-effective and highly active cocatalysts for overall water splitting.