Hydrogen production using plasmonic photocatalyst has attracted increasing attention since it improves light harvesting and photoefficiency. Herein, we have designed a plasmonic photocatalyst in a core–shell nanostructure that enabled an improvement of light harvesting and photocatalytic production of hydrogen using a very low amount of gold nanoparticles. The core–shell nanostructure was found to mimic the focusing of light observed for the lens-like epidermal cells. Thus, the core–shell nanostructure acts as a convex nanolens to reinforce the electromagnetic field at the nanostructure surface. The electric field was also found to be enhanced, which improves the energy absorbed for gold particles located in the core–shell nanostructure. Thus, by adjusting the diameter of the core–shell nanostructure, an optimal intensity for the localized surface plasmon resonance of gold was obtained. Tuning the size of the core–shell nanostructure enabled to improve the absorption at the reactive surface, thus increasing the photocatalytic hydrogen production efficiency by 5-fold.