Iron sulfides represent a reactive class of materials, and their structural diversity, phases and redox-active surfaces have a strong bearing on environmental processes. Iron-sulfide minerals occur widely in anoxic sediments and aquatic systems, where they take part in a range of biogeochemical reactions. Yet iron-sulfur chemistry is not confined to the mineral phases alone. Nanoparticulate FeS exists as a metastable, poorly crystalline form with high surface reactivity compared to bulk iron sulfide minerals, and this has drawn growing interest in FeS nanomaterials both as environmentally relevant reactive phases and as low-cost materials for remediation. This review compiles current knowledge on the chemistry, formation and reactivity of nanoparticulate FeS, with particular consideration to structural characteristics and to how nanoscale features such as particle size, surface coordination and crystallinity shape redox behavior. A major focus is to link synthesis routes, structure-reactivity relationships and environmental performance in one consolidated analysis. Advances in synthetic strategies, structural characterization and mechanistic studies are discussed with a focus on connecting FeS structure, electron transfer pathways and environmental reactivity. Recent applications of FeS nanomaterials in contaminant remediation are also evaluated, including reduction of toxic metals and catalytic transformation of pollutants, alongside the practical challenges such as particle instability, oxidation, and aggregation in natural systems that still limit their use. Nanoparticulate FeS shows real promise for environmental remediation, but stability issues and gaps in mechanistic understanding continue to constrain their practical deployment, pointing to a need for better material design and more standardized evaluation under environmentally relevant conditions.