Currently available methods to stably disperse iron oxide nanoparticles (IONPs) in aqueous solution need to be improved due to potential aggregation, reduction of superparamagnetism, and the use of toxic reagents. Herein, we present a facile strategy for aqueous transfer and dispersion of organic-synthesized IONPs using only polyethylene glycol (PEG), a biocompatible polymer. A library of PEG derivatives was screened, and it was determined that amine-functionalized six-armed PEG, 6(PEG-NH(2)), was the most effective dispersion agent. The 6(PEG-NH(2))-modified IONPs (IONP-6PEG) were stable after extensive washing, exhibited high superparamagnetism, and could be used as a platform material for secondary surface functionalization with bioactive polymers. IONP-6PEG biofunctionalized with hyaluronic acid (IONP-6PEG-HA) was shown to specifically label mesenthymal stem cells and demonstrate MR contrast potential with high r(2) relaxivity (442.7 s(-1)mM(-1)) compared to the commercially available Feridex (182.1 s(-1)mM(-1)).