Methanosarcina acetivorans is a strictly anaerobic non-motile methane-producing Archaea expressing protoglobin (Pgb) which might either facilitate O2 detoxification or act as a CO sensor/supplier in methanogenesis. Unusually, M. acetivorans Pgb (MaPgb) binds preferentially O2 rather than CO and displays anticooperativity in ligand binding. Here, kinetics and/or thermodynamics of ferric and ferrous MaPgb (MaPgb(III) and MaPgb(II), respectively) nitrosylation are reported. Data were obtained between pH 7.2 and 9.5, at 22.0 °C. Addition of NO to MaPgb(III) leads to the transient formation of MaPgb(III)NO in equilibrium with MaPgb(II)NO+. In turn, MaPgb(II)NO+ is converted to MaPgb(II) by OH−-based catalysis. Then, MaPgb(II) binds NO very rapidly leading to MaPgb(II)NO. The rate-limiting step for reductive nitrosylation of MaPgb(III) is represented by the OH−-mediated reduction of MaPgb(II)NO+ to MaPgb(II). Present results highlight the potential role of MaPgb in scavenging of reactive nitrogen and oxygen species.