Plasmonic enhancement is a versatile and convenient way to enhance the conversion efficiency of various photoenergy conversion systems, such as photocatalysts and solar cells. We refine a plasmonic enhancement system by focusing on a carrier blocking layer (between a plasmonic metal and a photoactive layer), which is commonly used to prevent a major quenching channel in a plasmonic enhancement system. The hydrogen evolution reaction (HER) activity is enhanced by 33 times from the introduction of a carrier-selective blocking layer (CSBL) in Ag-CdS nanoparticles. The Ag2S layer, a typical example of a CSBL, synergistically improves the plasmonic enhancement effect of Ag on the photocatalytic HER activity of CdS by both the selective blocking of photoexcited electrons and the effective transfer of holes, which extends the lifetime of the active species (electrons in the conduction band) in the semiconductor photocatalyst (CdS) to accelerate the photocatalytic HER. We propose a new strategy for a further improvement of plasmonic enhancement systems.