Photocatalytic hydrogenation of nitroaromatics was carried out with silica-supported iron oxides (FeOx/SiO2) as catalysts and hydrazine monohydrate as a reductant. Photoirradiation of the catalysts at lambda > 300 nm promotes hydrogenation of nitroaromatics to the corresponding anilines with more than 90% selectivity. The FeOx/SiO2 catalysts with lower Fe loadings possess monomeric Fe-III species behaving as the active sites and exhibit higher photocatalytic activity and aniline selectivity. Electron spin resonance (ESR) analysis revealed that photoexcitation of the hydrazine-adsorbed monomeric Fe-III species produces Fe-II species by the reduction with hydrazine. The Fe-II species promote the reduction of nitroaromatics to anilines, along with the Fe-II -> Fe-III oxidation.