Magnetic iron oxide nanoparticles have been proven to have versatile applications in biomedicine. Although numerous strategies have been developed to synthesize hydrophilic magnetic nanoparticles, there is still a challenge in the quantity and controllability of preparation of highly dispersible, stably water-dispersive magnetic nanoparticles. The current work presents a deep-eutectic solvent electrolysis to synthesize magnetic nanoparticles. In the electrolysis process, iron atoms at the anode electrode are oxidized to ferric ions, and then the ferric ions are combined with reactive oxygen species that derived from the decomposition of deep-eutectic solvents to form iron oxide nanocrystals. Concomitantly, hydrophilic radicals of amine groups produced by electrolyte decomposition are grafted on the particles. The monodisperse nanoparticle size ranged from 6 to 9 nm. The hydrophilic group loaded nanoparticles can be highly dispersed in water with neither surface post-modification nor organic stabilizers. The hydrodynamic particle diameter is between 20 and 30 nm. The transparent aqueous dispersions can be maintained for more than 600 days without precipitation.