In　the　present　work,　we　synthesized　and　characterized　an　electrocatalyst　consisting　of　sub-nanometric　Pt　clusters　uniformly　dispersed　on　a　TiO2　support.　X-ray　photoelectron　spectra　(XPS)　and　X-ray　adsorption　fine　structure　(XAFS)　data　demonstrate　that　these　sub-nanometric　Pt　clusters　are　in　a　highly　oxidized　state　and　possess　two　localized　Pt-O　coordination　structures.　The　Pt-O　bonds　between　the　oxidized　Pt　clusters　and　the　TiO2　give　rise　to　a　strong　metal-support　interaction　(SMSI).　When　applied　to　the　hydrogen　evolution　reaction　(HER),　this　catalyst　exhibits　significantly　enhanced　catalytic　activity　(increased　by　a　factor　of　up　to　8.4)　and　enhanced　stability　compared　with　the　state-of-the-art　commercial　Pt/C　catalysts.　Particularly,　the　additional　XPS　and　XAFS　characterizations　of　the　catalyst　after　long-term　electrolysis　demonstrate　the　absence　of　metallic　Pt　species,　confirming　that　the　catalytic　active　site　comes　from　the　oxidized　Pt　clusters　rather　than　from　the　the　metallic　Pt　species.　This　improved　performance　is　considered　to　be　induced　by　the　unique　electronic　structure　of　the　oxidized　Pt　clusters　and　by　the　SMSI.　Based　on　the　results　of　density　functional　theory　calculations,　the　5d　orbital　of　the　oxidized　Pt　cluster　atoms　appears　to　hybridize　with　the　H　1s　orbital　to　form　weak　Pt-H　valence　bonds,　leading　to　a　DG　(relative　free　energy)　value　of　approximately　zero　eV　for　H*　absorption.　This　effect　explains　the　mechanism　responsible　for　the　excellent　catalytic　activity　of　these　oxidized　Pt　clusters　for　the　HER.　This　work　therefore　provides　important　insights　into　the　role　of　oxidized　Pt　clusters　as　an　HER　electrocatalyst.　The　evident　stabilization　of　the　oxidized　Pt　clusters　on　TiO2　supports　via　the　charge-transfer　mechanism　provides　a　useful　approach　for　improving　the　durability　of　electrocatalysts　that　may　be　applicable　to　other　noble　metal/support　systems.