For thermoelectric (TE) applications, the surface of exfoliated black phosphorus (BP) can be successfully functionalized with Au nanoparticles (NPs), leading to significantly enhanced TE performance for a controlled Au NP content. A facile and selective decoration of metal species on the defect sites of BP is achieved by the spontaneous formation of Au NPs on the surface of BP through a redox reaction of Au precursors. Such a heterostructure provided by the Au decoration of BP enhances electrical conductivity (from 0.001 to 63.3 S cm −1 ) through tuning of the charge carrier concentration and retains the initial Seebeck coefficient of BP. Consequently, the TE power factor of the Au-decorated BP increases significantly to 68.5 µV m −1 K −2 , which is 2740 times that of the pristine BP. More significantly, in contrast to the severe degradation of the pristine BP in the air, surface-functionalized BP exhibits excellent stability upon exposure to air for a long period, which is beneficial for practical TE applications. Given these interesting and unique properties of Au-decorated BP, a vertical TE generator with a high power output of 79.3 nW (ΔT = 2 °C) is prepared by using the Au-decorated BP as a p-type TE material.