Nanoparticles (NPs) and their fabrication routes are intensely studied for their wide range of application in optics, chemistry, and medicine. Γ-ray and ion irradiation of precursor matter are established methods that facilitate tailored NP synthesis without complicated chemistry. Here, we develop and explore NP synthesis based on irradiating precursor microparticles with low-energy electron beams. We specifically demonstrate the fabrication of plasmonic gold nanoparticles of sizes between 3 and 350 nm on an amorphous SiO_x substrate using a 30 kV electron beam. By detailed comparison with electron scattering simulations and thermodynamic modeling, we reveal the dominant role of inelastic electron-matter interaction and subsequent localized heating for the observed vaporization of the precursor gold microparticles. This general principle suggests the suitability of electron-beam irradiation for synthesizing NPs of a wide class of materials.