Dotarem is the most used gadolinium-based contrast agent and one of the main sources of anthropogenic gadolinium in water systems. A Dotarem aqueous solution at its excretion level concentration was photocatalyzed by different materials. A 42% conversion was reached by photolysis, followed by photocatalysis with TiO2, iron oxides, and TiO2/γ-Fe2O3 (39–29%). Transformation products were identified to correspond to hydroxylated-Dotarem derivatives, Gd-free compounds, and Fe-complexes. A conversion pathway was proposed to demonstrate that, under UVC irradiation, Dotarem released toxic Gd3+, and in the presence of Fe-containing materials, transmetallation reactions occurred between the Gd3+ and Fe3+ ions.