Plasmonic metal nanostructures have been widely applied in photodetectors for the enhanced light response range and sensitivity. In contrast, photodetection based on surface plasmon effect of the emerging plasmonic nonmetals has not been investigated. Here, single nonmetal plasmonic WO_3−x nanosheets were used as the sensing material for UV–Vis–NIR broad spectral photodetectors. The plasmonic WO_3−x nanosheets were synthesized by solvothermal and follow-up thermal treatment in a hydrogen-containing atmosphere, which exhibited a localized surface plasmon resonance (LSPR) band centered at 899 nm with broad spectral absorption spanned from UV to NIR. Then photodetectors fabricated facilely by depositing Au electrodes on a film of WO_3−x nanosheets showed sensitive response for the regulation of conductance through the plasmonic hot free charge carriers. The responsivity and detectivity were 52 mA/W and 1.46 × 10⁸ Jones under an incident light with a wavelength of 980 nm with an ultralow bias of 0.01 V, and went up to 538 mA/W and 4.75 × 10⁸ Jones under 0.1 V. The results demonstrate the great potential of nonmetal plasmonic materials for photodetection.