Superparamagnetic nanoparticles are being actively developed for various applications in oil fields. A key requirement for such nanoparticles is the long-term stability of their dispersion under harsh subsurface conditions such as high salinity and high temperature. In this study, through functionalization of the surface of the magnetite nanoparticles with a sulfonated phenolic resin, stable dispersions of the nanoparticles in 5 wt % NaCl solution at neutral to basic pH were achieved. Sulfonated phenolic resin is aromatic in nature and can endure high temperatures; moreover, the strongly anionic sulfonate groups facilitate sufficient electrostatic repulsion between the particles even in a highly saline environment. Sulfonated phenolic resin was synthesized by heating a mixture of 4-hydroxybenzenesulfonic acid and formaldehyde at 1:1 molar ratio. Then, by synthesizing magnetite nanoparticles by a co-precipitation process in a solution of sulfonated phenolic resin, one-pot synthesis/surface functionalization was accomplished, which not only simplifies the manufacturing process but also can potentially lower the cost of production owing to the use of readily available phenolic resins. The reliability of the technique was confirmed by the detailed characterization of the nanoparticle through size distribution analysis, thermogravimetric analysis, zeta-potential measurements, transmission electron microscopy imaging, and energy-dispersive X-ray spectroscopy mapping. Further, Langevin curve was generated for evaluating the superparamagnetism of the nanoparticles.