The successful treatment of a disease via individualized treatment protocols relies on an early and accurate diagnosis. Advances to imaging hardware, such as hybrid PET/MRI scanners, have overcome the inherit disadvantages associated with the individual imaging modality. However, well-designed multimodal contrast agents are essential to optimally exploit hybrid PET/MRI systems. Herein, we show that core-cross-linked azide-functional star polymer nanoparticles can be simultaneously labelled with a radioisotope (radioiodine) and a clinically-used MRI contrast agent (Gd-DOTA) by exploiting an elegant copper-catalyzed one-pot three-component reaction creating an iodotriazole. The nanoparticles have a longitudinal relaxivity of 5.7 mM-1 s-1 at 7 T (as compared to 3.8 mM-1 s-1 for commercially available Gd-DTPA), and a radiochemical yield of 58% was achieved. Furthermore, we show that the radioiodine content can be fine-tuned without affecting the final Gd-DOTA loading. While we have demonstrated the versatility of the approach with 125I, an isotope widely used in biological research, the availability of various radioiodine isotopes enables potential applications in SPECT (123I), PET (124I) and in theranostics by combining radioimmunotherapy (131I) with MRI.