Metal　nanoparticle　(NP)　cocatalysts　are　widely　investigated　for　their　ability　to　enhance　the　performance　of　photocatalytic　materials;　however,　their　practical　application　is　often　limited　by　the　inherent　instability　under　light　irradiation.　This　challenge　has　catalyzed　interest　in　exploring　high-entropy　alloys　(HEAs),　which,　with　their　increased　entropy　and　lower　Gibbs　free　energy,　provide　superior　stability.　In　this　study,　3.5 nm-sized　noble-metal-free　NPs　composed　of　a　FeCoNiCuMn　HEA　are　successfully　synthesized.　With　theoretic　calculation　and　experiments,　the　electronic　structure　of　HEA　in　augmenting　the　catalytic　CO2　reduction　has　been　uncovered,　including　the　individual　roles　of　each　element　and　the　collective　synergistic　effects.　Then,　their　photocatalytic　CO2　reduction　capabilities　are　investigated　when　immobilized　on　TiO2.　HEA　NPs　significantly　enhance　the　CO2　photoreduction,　achieving　a　23-fold　increase　over　pristine　TiO2,　with　CO　and　CH4　production　rates　of　235.2　and　19.9 µmol　g−1　h−1,　respectively.　Meanwhile,　HEA　NPs　show　excellent　stability　under　simulated　solar　irradiation,　as　well　high-energy　X-ray　irradiation.　This　research　emphasizes　the　promising　role　of　HEA　NPs,　composed　of　earth-abundant　elements,　in　revolutionizing　the　field　of　photocatalysis.　©　2024　Wiley-VCH　GmbH.