More than 400 million tons of iron and steel slag is produced each year in the world. Most of the steel slag is recycled in the developed countries, mainly for use as construction aggregate, however excess steel slag is usually sent to landfills for disposal. Slag dumping can have an impact on the environment due to the release of harmful non-degradable elements present in the slag, which causes environmental problems such as water and soil pollution. A new way of recycling steel slag was approached by investigating its use as low cost precursor for the synthesis of iron oxide and oxyhydroxide nanoparticles. By chemical treatment of steel slag samples provided by the steel plant Arcelor Mittal Avilés, located in the North of Spain, were obtained two nanosized iron oxides with interesting properties and technological applications: magnetite (Fe3O4) and akaganéite (β-FeOOH). Characterization of the slag and both oxides was carried out by elemental chemical analysis, XRF, X-ray diffraction, N2 adsorption at 77K, thermal analysis and SEM and TEM microscopy.