Sub-bandgap near-infrared silicon (Si) photodetectors are key elements in integrated Si photonics. We demonstrate such a Si photodetector based on a black Si (b-Si)/Ag nanoparticles (Ag-NPs) Schottky junction. This photodetector synergistically employs the mechanisms of inner photoemission, light-trapping, and surface-plasmon-enhanced absorption to efficiently absorb the sub-bandgap light and generate a photocurrent. The b-Si/Ag-NPs sample was prepared by means of wet chemical etching. Compared to those of a planar-Si/Ag thin-film Schottky photodetector, the responsivities of the b-Si/Ag-NPs photodetector were greatly enhanced, being 0.277 and 0.226 mA/W at a reversely biased voltage of 3 V for 1319- and 1550-nm light, respectively.