Significance Mineral dust has long been considered an important source of Fe to the surface ocean, especially in remote areas away from continental margins. However, anthropogenic Fe derived from fossil-fuel burning may also contribute Fe to surface waters. Here we show in situ evidence of anthropogenic Fe in seawater, based on the concentrations and isotopic composition of Fe measured during a springtime cruise which transected the North Pacific Ocean. Our results suggest that anthropogenic Fe could play a globally important role in marine productivity and carbon cycling.