The speciation of inorganic nitrogen species in oxidized (nitrite and nitrate ions) and reduced (ammonium ion) forms is an important item for understanding of water environments. Flow injection analysis (FIA), capillary electrophoresis (CE), and ion chromatography (IC) are useful methods for simultaneous determination of those ions. However, it is difficult to determine the ions due to the interferences by matrix ions in seawater. In this study, simultaneous and selective determination of inorganic nitrogen species (nitrite, nitrate, and ammonium ions) in seawater samples was examined by ion chromatography with UV-VIS detection. Nitrite and nitrate ions were separated on monolith types of ODS columns equilibrated with DDAB or anion-exchange column with high anion-exchange capacity. Inorganic salt solutions such as potassium chloride and sodium chloride were used as eluents and nitrite and nitrates were detected by UV at 225 nm without any interference. Ammonium ion was detected at 710 nm as indophenol blue derivative by post-column reaction in the presence of dichloroisocyanurate and 1-naphthol. This IC system was useful for the determination in non-saline waters. However, the indophenol reaction was interfered with the matrix ions in seawater, mainly alkaline-earth cations. Thus, the use of cation exchange columns and the addition of chelating reagents such as sodium ethylenediaminetetraacetate and sodium citrate in the reaction solutions and samples were examined and optimized in order to suppress the interference by matrix ions in seawater samples. The obtained methods enabled the direct detection of inorganic nitrogen species and were applied to simultaneous and selective determination of inorganic nitrogen species in real seawater samples.