The ubiquitous distribution of the iron oxides makes them interesting materials for different fields. Iron oxides occur at the nanoscale in the environment such as in loess, peat bog, and paddy fields and in living organisms such as bacteria, animals, and plants. On the other hand, the engineered iron oxide nanoparticles (NPs) are materials of great interest in different technological applications, such as cancer diagnosis and therapy, drug delivery vehicles, resonance imaging, fabrication of biocompatible magnetic fluids, tunneling magnetoresistance, magnetic components, electrochromic devices, lithium-ion batteries, photoelectrochemical systems, and solar filters. In addition, the iron oxides can be used as nanoenzymes, which are nanostructured iron oxides mimicking the activity of diverse biological catalysts (enzymes). This chapter describes the peroxidase-like catalytic activity of different iron oxides and their application for water remediation. This chapter is organized as follows: A brief description of different iron oxides is given in the first part. Next, the surface properties of iron oxides are explained. Finally, the peroxidase-like catalytic activity of different iron oxides is described, focusing on the Fenton-like and photo-Fenton processes, which are of great value for the treatment of polluted water. Additionally, the improvements for the peroxidase-like catalytic activity of iron oxides are presented.