Carbon nanoparticles (CNPs) are attracting great attention as potential multifunctional agents for biomedical applications because of their bright fluorescence, low toxicity and flexibility of their physico-chemical properties. In the present paper, aqueous solutions of CNPs doped with gadolinium (Gd) (Gd-CNPs) within a widely varying range of Gd concentrations were prepared by hydrothermal synthesis. The influence of Gd doping on the optical properties and magnetic resonance (MR) relaxivity of Gd-CNPs was revealed. The Gd content was determined using X-ray fluorescence and spectrophotometry analysis. The composition of surface functional groups and coordination of Gd ions in Gd-CNPs were established by means of IR absorption spectroscopy and X-ray photoemission spectroscopy (XPS). The optical properties of Gd-CNPs in aqueous solutions were characterized by means of UV-visible-near-IR absorption spectroscopy and photoluminescence measurements with different excitation wavelengths. The local surroundings of Gd ions and paramagnetic centers in Gd-CNPs were probed by using electron paramagnetic resonance (EPR) spectroscopy. MR proton relaxation measurements in aqueous solutions of Gd-CNPs were carried out to determine the effect of Gd concentration on their MR contrasting. The obtained results characterize the coordination of Gd ions in Gd-CNPs and demonstrate new insights for controlling the optical and MR contrast properties of these nanoparticles for biomedical applications.