The development of synthetic materials that function well under complicated working conditions is of pivotal importance for many practical applications. In this work, we report a facile and universal method that imparts robust repair of thermoplastic materials in a wide range of working environments. The repair process takes advantage of the highly efficient photothermal effect enabled by monodispersed Fe₃O₄ nanoparticles incorporated into a number of thermoplastic polymers. We demonstrate that the robust thermogenesis not only allows the polymers to be repaired under the conditions of scratching, rupture and fragmentation in air, but also allows for rapid in situ repair of material defects underwater. This method is versatile, straightforward and opens up a novel practical route for material repair, especially for the repair of underwater coatings and components.