Hydrothermal methods have conventionally enabled the synthesis of a wide range of nanomaterials. However, these simple, single-step syntheses lack scalability due to the need of high temperatures and autogenous pressures to enable the dissolution of reagents and crystallisation of the product. In this work, we demonstrate for the first time fast continuous synthesis of ceria nanoparticles at moderate conditions through the combination of the deep eutectic solvent reline (an eutectic mixture of choline chloride and urea) as reaction medium and the high heat and mass transfer rate offered by microreactors. Almost 100% yields are obtained within 100 s of residence time at 160°C, with some conversion achieved even at temperatures as low as 120°C. Such rapid synthesis takes place thanks to the molecular structure of the solvent which facilitates the fast nucleation of cerium oxycarbonate as an intermediate product. As expected in a kinetically controlled system, pressure and initial cerium concentration have negligible effects on the yield obtained. The rapid reaction, the cheap, benign and environmentally friendly solvent, and the lack of additional additives in this work opens the door to sustainable large-scale continuous synthesis of ceria nanoparticles as well as other nanostructured materials.