The synthesis of superparamagnetic nanoclusters is critical for ultra-sensitive magnetic resonance imaging (MRI). Herein, we describe the synthesis of water-soluble, biocompatible and superparamagnetic gadolinium-doped iron oxide nanoclusters (GdIO NCs) via a one-pot reaction by thermal decomposition of ferric oleate and gadolinium oleate precursors with α,ω-dicarboxyl poly(ethylene glycol) as a surfactant. The resulting water-dispersible GdIO NCs possess good stability and monodispersity with narrow size distribution, and exhibit superparamagnetic behaviors. We also explored the effect of gadolinium doping amounts on the magnetic properties and longitudinal (r1) and transverse relaxivity (r2) of the nanoclusters. In addition, the GdIO NCs can be functionalized with fluorescein isothiocyanate (FITC) while maintaining their magnetic properties and biocompatibility. The GdIO NCs and FITC conjugated NCs were preliminarily evaluated as MRI and fluorescent probes. The results show that the GdIO NCs provide an important nano-platform for theranostics with non-invasive MRI and optical monitoring capabilities.