A reversible addition-fragmentation chain transfer (RAFT) agent was directly anchored onto superparamagnetic Fe3O4 nanoparticles (SPNPs) in a simple procedure using a ligand exchange reaction of 2-[(dodecylsulfanylcarbonylthiolsulfanyl) propionic acid] (DCPA) with oleic acid initially present on the surface of Fe3O4 nanoparticles. The DCPA-modified SPNPs were then used for the surface-mediated RAFT polymerization of di(ethylene glycol) ethyl ether acrylate and (oligoethylene glycol) methyl ether acrylate to fabricate structurally well-defined hybrid SPNPs with temperature-responsive poly[di(ethylene glycol) ethyl ether acrylate-co-(oligoethylene glycol) methyl ether acrylate] shell and magnetic Fe3O4 core. Evidence of a well-controlled surface-mediated RAFT polymerization was gained from a linear increase of number-average molecular weight with overall monomer conversions and relatively narrow polydispersity indices of the copolymers grown from the SPNPs. The resultant hybrid nanoparticles exhibited superparamagnetic property with a saturation magnetization of 55.1–19.4 emu/g and showed a temperature-responsive phenomenon as the temperature changed between 25 and 40 °C. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3420–3428