[en] Highlights: • An alkali modification process produces highly dispersed ultrafine Pt nanoclusters. • Ultrafine Pt nanoclusters have metallic Pt⁰ and oxidized Pt²⁺ species. • NYTiO₂ as a model photocatalyst for H₂ production. • NYTiO₂-Pt with Pt⁰ and Pt²⁺ species as light harvesting reactor for efficient H₂ production. We demonstrate an alkali modification process to produce highly dispersed ultrafine Pt nanoclusters with metallic Pt⁰ and oxidized Pt²⁺ species as co-catalyst anchored on nanosheet-constructed yolk-shell TiO₂ (NYTiO₂-Pt) acting as light harvesting reactor for highly efficient photocatalytic H₂ production. Benefiting from the high surface area, highly dispersed ultrafine Pt nanoclusters (~0.6 nm) with Pt⁰ and Pt²⁺ species and special nanosheet-constructed yolk-shell structure, this novel light harvesting reactor exhibits excellent performance for photocatalytic H₂ production. The NYTiO₂-Pt-0.5 (0.188 wt% Pt) demonstrates an unprecedentedly high H₂ evolution rate of 20.88 mmol h⁻¹ g⁻¹ with excellent photocatalytic stability, which is 87 times than that of NYTiO₂-Pt-3.0 (0.24 mmol h⁻¹ g⁻¹, 1.88 wt% Pt), and also much higher than those of other TiO₂ nanostructures with the same Pt content. Such H₂ evolution rate is the highest reported for photocatalytic H₂ production with such a low Pt content under simulated solar light. Our strategy here suggests that via alkali modifying the photocatalysts, we can not only enhance the H₂ production for solar energy conversion but also significantly decrease the noble metal content for cost saving.