We describe the use of seawater as fuel to propel Janus micromotors. The new micromotors consist of biodegradable and environmentally friendly magnesium microparticles and a nickel-gold bilayer patch for magnetic guidance and surface modification. Such seawater-driven micromotors, which utilize macrogalvanic corrosion and chloride pitting corrosion processes, eliminate the need for external fuels to offer efficient and prolonged propulsion towards diverse applications in aquatic environments.