Graphitic carbon nitride modified with plasmonic Ag@SiO2 core–shell nanoparticles (g-C3N4/Ag@SiO2) are proposed for enhanced photocatalytic solar hydrogen evolution under visible light. Nanosized gaps between the plasmonic Ag nanoparticles (NPs) and g-C3N4 are created and precisely modulated to be 8, 12, 17, and 21 nm by coating SiO2 shells on the Ag NPs. The optimized photocatalytic hydrogen production activity for g-C3N4/Ag@SiO2 is achieved with a nanogap of 12 nm (11.4 μmol h−1) to be more than twice as high as that of pure g-C3N4 (5.6 μmol h−1). The plasmon resonance energy transfer (PRET) effect of Ag NPs is innovatively proved from a physical view on polymer semiconductors for photoredox catalysis. The PRET effect favors the charge carrier separation by inducing electron–hole pairs efficiently formed in the near-surface region of g-C3N4. Furthermore, via engineering the width of the nanogap, the PRET and energy-loss Förster resonance energy transfer processes are perfectly balanced, resulting in considerable enhancement of photocatalytic hydrogen production activity over the g-C3N4/Ag@SiO2 plasmonic photocatalyst.