Selenium nanoparticles (SeNPs) have been suggested for various applications such as food, animal feed, and biomedicine. However, thus far, wet chemical synthesis cannot produce sufficient quantities of SeNPs at the required quality. In this study, reactive aerosol processing is employed to produce SeNPs as powder concentrates of 2.2 to 4.0% (dry weight basis) on the scale of hundreds of kilograms. Citric acid is the most efficient organic precursor (92.5% SeNPs), but polydisperse particles are obtained, and some unknown dissolved Se species are present in the final product. Glycine is less efficient (82.2% SeNPs); however, monodisperse nanoparticles (153 nm; D-90 = 215 nm) are obtained, with selenite being the only residue in the final product. The obtained particles consisted of X-ray amorphous (i.e., not diffracting) Se, since no elemental Se allotrope or other solid Se phases were indicated in X-ray diffraction. Reactive aerosol processing involves a limited number of preparation steps (i.e., dissolution and atomization), uses only food-/feed-approved ingredients (selenite, citric acid, glycine) and allows continuous operation and zero waste generation. This makes reactive aerosol processing ideal for the large-scale production of SeNPs.