High-voltage power transmission in electrical grids requires reliable and durable dielectric polymers for wire insulation(1,2). Electrical treeing caused by high, local electric fields is a damaging process that leads to structure degradation and electrical conduction of dielectric materials, and ultimately, to catastrophic failure of the devices(3-5). Here, we demonstrate that the addition of less than 0.1 volume per cent of superparamagnetic nanoparticles into a thermoplastic polymer enables the repair of regions damaged by electrical treeing and the restoration of the insulating properties. Under the application of an oscillating magnetic field, the embedded nanoparticles migrate to the electrical trees and generate a higher local temperature, which heals the electrical tree channels in the polymer. Our method allows us to regenerate the dielectric strength and electrical resistivity over multiple cycles of tree formation and healing, which could be used to increase the lifespan and sustainability of power cables for electronics and energy applications.