The biocompatible-coated iron oxide nanoparticles (IONs) have attracted a great interest because of their various applications in biological science and medicine. In most cases, the toxic effect of naked iron oxide nanoparticles is completely cleared by adding a biocompatible coating, such as polysaccharides, polyethylene glycol (PEG), or biosynthesis of biocompatible-coated IONs using microorganisms such as bacteria. In the present study, polysaccharide-coated iron oxide nanoparticles were produced by a strain of Staphylococcus warneri isolated from a thermal spring. For identification of the isolated bacterium, 16S rRNA gene sequencing was done. Characterization of the nanoparticles was performed for the first time, using transmission electron microscopy (TEM), dynamic light scattering (DLS), thermogravimetric analysis (TGA), X-ray crystallography (XRD), Fourier-transform infrared (FTIR) spectroscopy, vibrating sample magnetometer (VSM), and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Results indicated that the spherical iron oxide nanoparticles were coated by a polysaccharide (13.6%), which provided a large negative charge of -91 mV and very low saturation magnetization of around 0.28 emu/g. The result of MTT assay on MOLT-4 cell lines showed that the percentage of viability was between 95.6% and 68.9% in the 10-100 µM of nanoparticle concentrations with a high IC value, which makes it appropriate for biomedical applications such as cancer therapy.