AbstractIn recent years, broad interest has arisen for solar‐driven evaporation, a promising technology for next‐generation desalination and sewage treatment. Efficient photothermal materials are the keys in solar‐driven evaporation systems, but they are usually involved in tedious preparation and easy damage. Herein, a magnet‐dominated photothermal device (MPD) with spiny surface has been developed by the direct assembly of magnetic nanoparticles in action of magnetic field. Such one‐step fabrication of photothermal layer avoids the use of harmful reagent and the consumption of pure water. The adjustable spiny morphology not only benefits to the light absorption via multiple reflection, but can facilely recover from damage. Water evaporation under solar irradiation can be accelerated by MPD to over 1.70 kg m⁻² h⁻¹, and keeps stable in a long‐term desalination. Without complex fabrication and high price, MPD could be considered as a rewarding attempt for the development of novel photothermal materials in water purification.image