Methanocaldococcus jannaschii(Mj), a hyperthermophilic and evolutionarily deeply rooted methanogenic archaeon from a deep-sea hydrothermal vent, produces F420-dependent sulphite reductase (Fsr) in response to exposure to sulphite. This enzyme allowsMjto detoxify sulphite, a potent inhibitor of methyl coenzyme-M reductase (Mcr), by reducing it to sulphide with reduced coenzyme F420(F420H2) as an electron donor; Mcr is essential for energy production for a methanogen. Fsr allowsMjto utilize sulphite as a sulphur source. Nitrite is another potent inhibitor of Mcr and is toxic to methanogens. It is reduced by most sulphite reductases. In this study, we report thatMjFsr reduced nitrite to ammonia with F420H2with physiologically relevantKmvalues (nitrite, 8.9 µM; F420H2, 9.7 µM). The enzyme also reduced hydroxylamine with aKmvalue of 112.4 µM, indicating that it was an intermediate in the reduction of nitrite to ammonia. These results open the possibility thatMjcould use nitrite as a nitrogen source if it is provided at a low concentration of the type that occurs in its habitat.