Here, a controlled synthesis of remarkable 3D photocatalysts is presented that is composed of ultrahigh-density unaggregated plasmonic Au nanoparticles (AuNPs) chemically bound to vertically aligned ZnO nanorod arrays (ZNA) through bifunctional molecular linkers. Experimental probes and electromagnetic simulations of electron transfer and localized plasmonic coupling processes are exploited to gain insight into the underlying light-irradiation-induced interactions in the 3D ZNA-AuNPs photocatalysts. Highly dense AuNPs on ZNA surfaces act as sinks for the storage of UV-generated electrons, which promote the separation of charge carriers and create numerous photocatalytic reaction centers. Furthermore, 3D finite-difference time domain simulation indicates that significant visible light confinement and enhancement around the ZNA-AuNPs interfacial plasmon "hot spots" contribute to efficient conversion of light energy to electron-hole pairs. Significantly, in comparison with the bare ZNA, the 10-nm-sized AuNPs-decorated ZNA exhibits 10.6-fold enhanced photoreaction rate in the entire UV-vis region. Moreover, various novel hybrid structures based on the plasmonic AuNPs and diverse nanostructures (films, powdered nanorods, mesoporous, and nanotubes) or functional materials (multiferroic BiFeO3, CuInGaSe2 absorber layers, and photoactive TiO2) are successfully constructed using the present synthesis methodology. It may stimulate the progress in materials science toward the synthesis of multifunctional plasmonic heterostructures or devices.