Theranostic nanoparticles offer the potential for mixing and matching disparate diagnostic and therapeutic functionalities within a single nanoparticle for the personalized treatment of diseases. In this article, we present composite iron oxide-gadolinium-containing Prussian blue nanoparticles (FeO@GdPB) as a novel theranostic agent for T-weighted magnetic resonance imaging (MRI) and photothermal therapy (PTT) of tumors. These particles combine the well-described properties and safety profiles of the constituent FeO nanoparticles and gadolinium-containing Prussian blue nanoparticles. The FeO@GdPB nanoparticles function both as effective MRI contrast agents and PTT agents as determined by characterizing studies performed in vitro and retain their properties in the presence of cells. Importantly, the FeO@GdPB nanoparticles function as effective MRI contrast agents in vivo by increasing signal:noise ratios in T-weighted scans of tumors and as effective PTT agents in vivo by decreasing tumor growth rates and increasing survival in an animal model of neuroblastoma. These findings demonstrate the potential of the FeO@GdPB nanoparticles to function as effective theranostic agents.