We use conventional and aberration-corrected transmission electron microscopy (TEM) and ab initio calculations to investigate the structural and electronic properties of β-FeSi2 nanoparticles, which are a promising material for photovoltaic applications due to a band gap of <1 eV and a high absorption coefficient. The nanoparticles have average sizes of ∼20 nm, form aggregates, and are prepared by gas-phase synthesis. Amorphous SiOx shells with thicknesses of ∼1.7 nm around β-FeSi2 cores are identified on individual nanoparticles using electron energy-loss spectroscopy, while stacking fault domains in the nanoparticles are observed using high-resolution TEM, nanobeam electron diffraction, and automated diffraction tomography. Ab initio calculations indicate only minor changes in band structure in the faulted structure when compared to perfect β-FeSi2. The optical properties of imperfect β-FeSi2 nanoparticles are therefore expected to be the same as those of the perfect structure, suggesting that β-FeSi2 nanoparticles are suitable candidates for use in optical absorber layers in thin film solar cells.