The reducing capability of two-dimensional black phosphorus is demonstrated. The high reducing ability and unique twodimensional morphology of black phosphorus not only facilitate in situ synthesis of Au nanoparticles and BP@Au composites, but also enable multiscale control of local reduction of GO to reduced GO (rGO). The novel two-dimensional reductant has large potential in various in situ nanofabrication applications. 1, 2 For instance, graphene boasting good optoelectronic characteristics and high mechanical strength has large potential in energy conversion and storage 3-5 and transition-metal dichalcogenides (TMDs) with unique semiconducting properties are attractive to scalable digital field-effect transistors.6-8 Furthermore, the chemical reactivity of 2D materials such as graphene oxide (GO) and TMDs has raised concerns and novel templates have been proposed to fabricate functional composites.