The present invention provides iron phosphide nanoparticles in which iron atoms are in a low valence state and which are stable under an atmospheric condition, a production method therefor, and a reduction catalyst. The present invention relates to iron phosphide nanoparticles having peaks at diffraction angles (20.5) of 48.3 and 32.7 in a powder X-ray diffraction measurement using CuK radiation, wherein, when the iron phosphide nanoparticles are measured by X-ray photoelectron spectroscopy (XPS), iron atoms contained therein have a peak in a range of 706.0 to 707.5 eV in an Fe2p.sub.3/2 spectrum.