A bioelectrochemical anaerobic ammonium oxidation via direct interspecies electron transfer in bulk solution was first found in an anaerobic reactor equipped with a pair of bioelectrode polarized at 0.6 V. The removal of ammonium along with a decrease in nitrite and alkalinity was observed when ammonium and nitrite are available as electron donor and acceptor, respectively, in a bioelectrochemical reactor. The requirements of nitrite nitrogen and alkalinity for the removal of ammonium nitrogen are around 0.58 mg NO₂N/mg NH₄N and 2.0 mg as CaCO₃/mg NH₄N, respectively. The bioelectrochemical ammonium oxidation does not produce nitrate as a by-product as well. The microbial groups involved in bioelectrochemical ammonium oxidation are electroactive autotrophs and can be enriched from anaerobic digestion sludge from sewage treatment plant by the polarized electrode. This bioelectrochemical ammonium oxidation is a novel approach recommended for treatment of nitrogen-rich wastewater.