The spinning disc reactor (SDR) is used for the size-selective synthesis and functionalization of nanoscale superparamagnetic iron oxide particles in a single-step continuous-flow process. Nanometer-sized magnetite (Fe3O4) particles are precipitated in a highly sheared thin liquid film on a rapidly rotating disc. A novel liquid-liquid synthesis is presented whereas control over particle size is reached via adjustable micromixing properties on the disc. The nanoparticles with tailored size, narrow size distribution, and high saturation magnetization can thus meet the special requirements for the use in biotechnological, medical, and catalytic applications. The functionalization of the particles with oleic acid for higher biocompatibility and better dispersibility is accomplished in the SDR in a single step.