A novel and facile methodology for the in situ surface functionalization of Fe3O4 nanoparticles is proposed, based on the use of aryl diazonium salts chemistry. The grafting reaction involves the formation of diazoates in a basic medium. These species are unstable and dediazonize along a homolytic pathway to give aryl radicals which further react with the Fe3O4 NPs during their formation and stop their growth. Advantages of the present approach rely not only on the simplicity, rapidity, and efficiency of the procedure but also on the formation of strong Fe3O4-aryl surface bonds, highly suitable for further applications.