Magnetic resonance imaging (MRI) contrast can be enhanced through the use of magnetic nanoparticles. These nanoparticles alter the relaxation time of ¹H nuclei in water molecules that are present in tissues, providing sharper and more detailed images. The use of natural polymers such as sodium alginate, in addition to these being biocompatible and non-toxic, will ensure greater colloidal stability of the suspension, allowing for its use as a contrast agent for MRI. Therefore, this study aimed to prepare and analyze the behavior of iron oxide nanoparticles (FeNPs) to assess their potential application as a contrast agent for MRI diagnosis. FeNPs were prepared in an aqueous medium using the co-precipitation method. Subsequently, the surface of the nanoparticles was coated with different concentrations of sodium alginate (2.5, 5.0, 7.5, and 10.0 mg.mL^-1) to make FeNPs stable in an aqueous environment, as well as biocompatible. The efficiency of FeNPs (with and without alginate) as contrast agents was evaluated through relaxivity measurements (20 MHz at 25 ºC). The obtained results showed that with the addition of alginate, FeNPs showed a decrease in transverse relaxation time (T₂) compared to NPs without the polymer. These results may indicate that the incorporation of the stabilizer led to a change in the mobility of water molecules, thereby altering the diffusion time of water molecules near the superparamagnetic center and increasing the colloidal stability of iron oxide nanoparticles in a suspension. Thus, based on the obtained results, FeNP alginates show potential for use as biocompatible contrast agents for diagnostic imaging.