In the last decades, the advent of nanotechnology has driven the study and application of nanoscale versions of magnetic materials. Among the various nanoparticles under research, iron oxide magnetic nanoparticles (MNP), namely iron oxides magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃), have attracted particular interest due to their superparamagnetism, biocompatibility and biodegradability. MNP are thus ideal platforms to work on a cellular and molecular level in several biomedical applications. In particular, the use of MNP as contrast agents for biomedical imaging through Magnetic Resonance Imaging (MRI) has been explored extensively in the last 30 years, taking advantage of the versatility of MNP functionalization due to the available large surface-to-volume ratio. Polymers, either synthetic or natural, are the most common class of materials employed as coatings for MNP, allowing to customize nanoprobes properties such as size, shape, magnetic relaxation, as well as cell-nanoprobe interactions (for example, specificity towards tissue types, responsiveness to cellular environment features), therapeutic effects or combination with other imaging modalities. While most biopolymers have intrinsic biocompatibility and biodegradability properties and are greener products, synthetic polymers offer engineering versatility and possibility of being tailor-made with specific properties. This review covers the properties of nanoscale iron oxides, production and stabilization methods of such nanoparticles, and their biomedical applications, mainly focusing on the engineering of polymeric-MNP assemblies towards the development of new hybrid magnetic-polymeric MRI nanoprobes.