Although single-atomic-layered alloys are regarded as promising components for improving the performance of broadband photodetectors, the origin of their enhanced photoresponsivity due to the introduction of dopants in the crystal lattice has not yet been investigated in depth. Herein, we comprehensively analyze the nature of the photoconductivity of a photodetector based on a niobium (Nb)-doped WSe2 monolayer. The Nb-doped WSe2 photodetector exhibited superior responsivity and specific detectivity compared with those of the undoped WSe2 photodetectors. Experimental and density functional theory analyses revealed that the introduction of Nb not only modified the Fermi level of WSe2 but also generated multiple electron-trapping sites, thereby increasing both the photovoltaic and photogating effects to improve the photocurrent of the device. We believe that this research presents a sophisticated approach to achieve photodetectors based on single-atomic-layered alloys, which are both highly sensitive and energy-efficient.