Coupling the plasmonic metals with semiconductors often induces strong charge and energy transfer across heterointerfaces, offering an unprecedented opportunity to break the fundamental limit of semiconductor optoelectronic devices. Herein, we demonstrate a broadened photodetection bandwidth with drastically enhanced photoresponsivity of photoelectrochemical cells by coupling the plasmonic–platinum nanoparticles with p-type AlGaN-semiconductor nanostructures. Benefiting from the localized surface plasmon resonance at the platinum-AlGaN nanostructure interface, our devices exhibit a striking 3 orders of magnitude boost of the photoresponsivity in the visible band, which is barely attainable in pristine wide band gap semiconductors. Simultaneously, a nearly sevenfold enhancement of the photoresponsivity can also be achieved under 254 nm light illumination, demonstrating high-responsive deep ultraviolet-sensitive broad-bandwidth photodetection. Most importantly, the proposed plasmon-induced metal/semiconductor hybrid nanoarchitectures, by embracing a diversity of plasmonic metals combined with the wide tunable band gap of the group III-nitride semiconductors via synergy of the plasmonic–photoelectric effect, show significant promise in designing specific wavelength-dominance broadband photosensing systems of the future.