MRI-contrasting agents based on paramagnetic molecules and superparamagnetic nanoparticles substantially enhance the resolution and stimulate developments of new modalities of MRI bioimaging. Endogenous magnetic materials produced in vivo in the organism represent a new class of promising contrasting agents. In particular, the metalloprotein ferritin, which is a basic protein responsible for accumulation of iron in the organism, is of the utmost interest as a molecular MRI contrast agent due to the presence of a paramagnetic oxide core. Despite the fact that the structure and magnetic properties of ferritin are well described in the literature, the information on the MRI contrasting properties of ferrihydrite nanoparticles, which are the main constituent of its core, is almost unavailable. In this work, we fill this gap and study the MRI contrasting properties of specially synthesized paramagnetic ferrihydrite nanoparticles stabilized by a polymer shell for high stability in physiological media. We show that such nanoparticles, similar to ferritin, exhibit pronounced negative (T2-weighted) contrasting and can serve as a good model of natural ferritin for studying its MRI-related properties. The weak magnetic and good contrasting properties in combination with stability in physiological media make the developed nanoparticles a valuable control tool for investigation of strongly magnetic nanoparticles. Furthermore, due to low toxicity, biocompatibility and possibility of functionalization with various receptors, the particles are promising for MRI contrasting, improvement of bioimaging techniques, development of advanced biosensors and next-generation multifunctional theranostic agents.