Four salts, SEALIFE (a synthetic sea salt), NaCl, Na2SO4, and NaCl+KCl, were applied to monitor the effects of salinity on “Candidatus Scalindua sp.”, a marine anaerobic ammonium oxidation (anammox) bacterium. The highest ammonium consumption of 10 μmol mg protein–1 d–1 was observed at 88 mmol L–1 of Na in the presence of NaCl. The highest inorganic carbon uptake of 0.6 μmol mg protein–1 d–1 was observed at 117 mmol L–1 of Na and at 16 mmol L–1 of K in the presence of NaCl+KCl. Thus, Na and K are both important for maintaining a high growth rate of “Candidatus Scalindua sp.”