Ammonia-oxidizing archaea (AOA) are one of the most abundant groups of microbes in the world’s oceans and are key players in the nitrogen cycle. Their energy metabolism—the oxidation of ammonia to nitrite—requires oxygen. Nevertheless, AOA are abundant in environments where oxygen is undetectable. By carrying out incubations for which oxygen concentrations were resolved to the nanomolar range, we show that after oxygen depletion, Nitrosopumilus maritimus produces dinitrogen and oxygen, which is used for ammonia oxidation. The pathway is not completely resolved but likely has nitric oxide and nitrous oxide as key intermediates. N. maritimus joins a handful of organisms known to produce oxygen in the dark. On the basis of this ability, we reevaluate the role of N. maritimus in oxygen-depleted marine environments.