Hydrogen boride (HB) sheets have recently been synthesized as new two-dimensional materials composed of boron and hydrogen. This study reports a new role of HB sheets as reductants. Ultraviolet-visible spectroscopy was undertaken for a mixture of HB sheets with several metal ions in solution. The sheets had a redox potential between -0.277 and -0.257 V versus standard hydrogen electrode (SHE). We subsequently demonstrated the facile formation of HB sheet nanocomposites with metal nanoparticles using this reduction property.