Combining various imaging modalities often leads to complementary information and synergistic advantages. A trimodal long-circulating imaging agent tagged with radioactive, magnetic resonance, and fluorescence markers is able to combine the high sensitivity of SPECT with the high resolution of MRI over hours and days. The fluorescence marker helps to confirm the in vivo imaging information at the microscopic level, in the context of the tumor microenvironment. To make a trimodal long-circulating probe, high-molecular-weight hyperbranched polyglycerols (HPG) were modified with a suitable ligand for (111)In radiolabeling and Gd coordination, and additionally tagged with a fluorescent dye. The resulting radiopharmaceutical and contrast agent was nontoxic and hemocompatible. Measured radioactively, its total tumor uptake increased from 2.6% at 24 h to 7.3% at 72 h, which is twice the increase expected due to tumor growth in this time period. Both in vivo MRI and subsequent histological analyses of the same tumors confirmed maximum HPG accumulation at 3 days post injection. Furthermore, Gd-derivatized HPG has an excellent contrast enhancement on T1-weighted MRI at 10× lower molar concentrations than commercially available Galbumin. HPG derivatized with gadolinium, radioactivity, and fluorescence are thus long-circulating macromolecules with great potential for imaging of healthy and leaky blood vessels using overlapping multimodal approaches and for the passive targeting of tumors.