Iron oxide nanocrystals are of great scientific and technological interest. In this work, these materials are the starting point for producing a reactive nanoparticle whose surface resembles that of natural green rusts. Treatment of iron oxide nanoparticles with cysteamine leads to the reduction of iron and the formation of a brilliant green aqueous solution of nanocrystals rich in iron(II). These materials remained crystalline with magnetic and structural features of the original iron oxide. However, new low-angle X-ray diffraction peaks as well as vibrational features characteristic of cysteamine were found in the nanocrystalline product. X-ray absorption spectroscopy (XAS), X-ray photoemission (XPS) and Mössbauer spectroscopies indicated the presence of an iron(II)-rich phase with high sulfur content analogous to the iron–oxygen structures found in natural green rusts. Electron microscopy found that these structural components remained associated with the nonreduced iron oxide cores. These sulfur-rich analogs of natural green rusts are highly reactive and were able to rapidly degrade a model organic dye in water. This observation suggests possible actuation with a cysteamine treatment of inert and magnetic iron oxide particles at the point-of-use for environmental remediation.