In recent years, multimodal contrast agents have attracted considerable attention in biomedical imaging. This paper reports the development of a multimodal nanoprobe based on silica-coated gadolinium oxide nanoparticles (NPs) for T₁-enhanced magnetic resonance (MR) and multicolor optical imaging. MR relaxivity measurements showed that these core–shell NPs could generate strong positive contrast enhancement in T₁-weighted MR imaging (MRI). Owing to the nominally co-doped Eu³⁺ and Tb³⁺ ions in the Gd₂O₃ host, the synthesized nanoprobes could simultaneously emit blue, green and red fluorescence signals. Furthermore, the cytotoxicity results showed that a thin silica coating on the surface of gadolinium oxide NPs could increase the biocompatibility of the fabricated nanoprobes considerably. The excellent multicolor fluorescence and MRI functionalities of these nanoprobes could have potential biomedical applications.