During the last decade, superparamagnetic iron oxide nanoparticles (SPIONs) have attracted the scientific community’s interest due to the multitude of applications in many fields of biomedicine namely as contrast media in diagnosis or as carriers for targeted drug delivery. We investigated the immunotoxicity of hydrophilic and surface functionalized SPIONs in vitro against THP-1-derived macrophages. Superhydrophilic ultra-small SPIONs (diameter ~4 nm) were produced by "trimming" the surface coating of the as-prepared nanoparticles. Cytotoxicity was assessed using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Flow cytometry was employed to study the effect of SPIONs on the redox stage of the cells through the detection of reactive oxygen species (ROS) generation using 2',7'-dichlorofluorescin-diacetate (DCF-DA), the induction of apoptosis using Annexin V-FITC/Propidium Iodide (PI) staining and the estimation of the percentages of cell population in different phases of the cell cycle (PI staining). A dose-dependent reduction in cell viability was recorded after treatment for 24 hours, which was more evident at doses higher than 5 μg/ml. The cytotoxicity of SPIONs was accompanied by a steep increase in ROS production and apoptotic cell population. Cell cycle analysis revealed that SPIONS (5 μg/ml) arrested THP-1-derived macrophages at G0/G1 phase. Our results showed that the ultra-small SPIONs induced immunotoxic effects causing the death of THP-1-derived macrophages. Nonetheless, this novel synthetic approach allows proceeding to further modifications to produce improved SPIONs that can deliver their beneficial promises to the biomedical field.