UV/chlorine has attracted much attention in flue gas and water treatment fields in recent years. However, only a few studies highlighted gas pollutants removal performance of UV/chlorine method in real water. In this study, nitrogen oxide (NOx) removal using UV-irradiated electrolyzed seawater (UV/electrolyzed seawater) was explored under simulated seawater environment in a semi-batch photochemical reactor. Comparison experiments between UV/H2O2 and UV/chlorine in both freshwater and seawater systems were performed. Results showed that NO removal efficiency in UV/chlorine freshwater system was higher than those in UV/chlorine seawater system, due to radical scavenging effects of water matrices in seawater. Several strong reactive radicals (e.g., (OH)-O-center dot, Cl-center dot, (OCl)-O-center dot, ClOH center dot-) yielded from UV/chlorine system were considered to contribute to NOx removal, among which the role of (OH)-O-center dot radical was confirmed by radical-trapping experiments. The alkalinity of the seawater adversely impacted contributions of these reactive radicals to NOx removal, which caused a sharp drop in NOx removal efficiencies. Comparatively, the chloride ion in seawater had slight influences on NOx removal efficiencies of UV/electrolyzed seawater. Then, optimization of NOx removal efficiencies against three key variables (active chlorine concentration, UV irradiation dose, and initial pH) was investigated in simulated seawater using Design Expert Software. A regression model of NOx removal efficiency was obtained with satisfactory accuracy. Results indicated that active chlorine concentration and UV irradiation dose significantly influenced NOx removal efficiencies, compared with pH effects. Furthermore, NOx removal mechanisms involved in reactive radicals were discussed for UV/electrolyzed seawater method. Due to pH-dependent characteristics of both active chlorine species and carbonate species in electrolyzed seawater, NOx removal pathways by UV/electrolyzed seawater were different as the environment changed from acid to alkalinity.