Ironmaking is one of the largest single causes of global warming. The reduction of iron ores with fossil fuel currently generates about 7% of the global carbon dioxide emissions for producing 1.85 billion tons of steel per year. This fact leads to an urgent decarbonization challenge in the steel industry and fuels efforts to deploy renewable and carbon-free reductants. Here, we show a novel approach to making sustainable steel by reducing iron ores with ammonia. Ammonia is an annually 180-million-ton-traded chemical energy carrier, with established transcontinental logistics and low liquefaction costs. It can be synthesized with green hydrogen, and release hydrogen again through the reduction reaction. This advantage connects it with green ironmaking as a promising hydrogen carrier, for replacing fossil reductants. The ammonia-based reduction of iron ores proceeds through an autocatalytic reaction; is kinetically as effective as hydrogen-based direct reduction; yields the same metallization; and can be industrially realized with existing technologies.