Subnanocluster　(SNC)　cocatalysts　are　considered　to　be　promising　cocatalysts　for　photocatalytic　hydrogen　evolution　reaction　(HER)　due　to　their　rich　surface-active　sites　and　high　atom　utilization　efficiency.　Herein,　a　reductant-mediated　in　situ　growth　of　the　atomic　clusters　procedure　is　developed　to　synthesize　SNC-Cu　serving　as　advanced　cocatalysts　on　TiO2　nanoparticles　for　HER.　The　optimized　SNC-Cu/TiO2　catalyst,　with　1.8　wt　%　metal　mass　loading,　exhibits　a　remarkable　hydrogen　evolution　rate　of　6832　μmol　g-1　h-1,　which　is　ca.　25-fold　that　of　bare　TiO2　(273　μmol　g-1　h-1　and　comparable　to　that　of　1　wt　%　Pt-modified　TiO2　(7754　μmol　g-1　h-1).　Meanwhile,　decent　photostability　and　recyclability　are　also　certified　via　a　20　h　durability　test.　Comprehensive　electron　microscopy　and　spectroscopy　characterizations　unveil　that　the　outstanding　performance　of　SNC-Cu/TiO2　can　be　attributed　to　accelerated　charge　separation/transfer　and　suppressed　photoinduced　electron-hole　recombination.　Furthermore,　in　situ　X-ray　photoelectron　spectroscopy　and　first-principle　calculations　reveal　that　the　electron-localized　SNC-Cu,　with　its　abundant　surface　sites,　facilitates　*H　adsorption　and　desorption,　thus　enhancing　the　HER　activity　significantly.　This　work　highlights　the　promising　potential　of　constructing　noble-metal-free　SNCs　in　semiconductors　for　photocatalytic　HER.　©　2024　American　Chemical　Society.