Magnetic iron oxide nanoparticles are frequently utilized as contrast agents in magnetic resonance imaging (MRI). However, the release of iron ions leads to the formation of reactive oxygen species (ROS), resulting in cell damage. In this study, we developed MRI contrast agents containing amine-functionalized MnFeO nanoparticles with a Prussian blue (PB) surface coating to enhance their biocompatibility. The prepared MNPs were embedded in polyvinylpyrrolidone and dried. The resulting crystals can be redispersed in water immediately before use, forming a stable colloid. The particle size of nanoparticles (43 ± 13 nm) is suitable for the intended application. The values of Hc (52 Oe) and Mr (3.7 emu/g) for the particles indicate a soft ferromagnetic nature. The coating of the particles with PB results in a significant reduction of their toxicity, as evidenced by a toxicological test on HEK293 cells. This colloid was tested in vitro as an MRI contrast agent as well as in healthy animal. The longitudinal relaxivity (r) of the PB-MnFeO-NH sample was determined to be 0.01 (mg/mL)ms. The transversal relaxivity was measured as well (r: 0.77 (mg/mL)ms and r*: 1.48 (mg/mL)ms, which were in the same range as Feraheme and Endorem. Prussian blue-coated MnFeO emerges as a promising T2-weighted contrast material, representing a novel combination of two well-known contrast-capable materials.