Iron oxide nanoparticles, due to their unique intrinsic magnetic properties, have attracted significant interest in biomedical applications. Therefore, the optimization of Magnetic Nanoparticles (MNPs) properties has always been the center of attention. For example, controlling the size of MNPs and enhancing their stability are the two most important parameters for biomedical applications that must be considered using an appropriate synthesis method. In this study, we synthesized bare magnetite NPs via an optimized coprecipitation route, followed by a post-synthesis procedure to coat them with polyethylene glycol (PEG). The synthesized MNPs presented optimized physiochemical properties, such as high stability, uniform size, relatively high saturation magnetization, and the absence of aggregation. The X-ray diffraction results revealed the high crystallinity of MNPs. Fourier-transform infrared studies confirmed the surface modification of particles with PEG. Moreover, an in vitro biological assay on the bacterial strain of Escherichia coli (E. coli) Rosetta (DE3) demonstrated low cytotoxicity and no evident cytotoxicity for bare MNPs and PEG-MNPs, respectively at a high concentration (0.5 mg/mL). Lastly, the potential of these MNPs was investigated as theranostic agents in magnetomotive ultrasound imaging (MMUS) and magnetic hyperthermia (MH). Based on the obtained results, both MNPs demonstrated the same performance in MMUS; however, PEG-coated MNPs showed a higher heating efficiency. These results show the great potential of the particles to be used as theranostic agents or other biomedical applications owing to their high saturation magnetization.