We proposed an indium tin oxide (ITO) sensor for detecting NH3 in seawater and carried out a series of experiments to investigate the feasibility of using it as a hazardous and noxious substance (HNS) sensor. The ITO layer revealed a distinct resistance change, which was linearly correlated with the NH3 concentration at room temperature. The ITO layer was evaluated using a seawater and ammonium hydroxide (NH4OH) solution. The evaluation revealed an almost linear output signal up to an NH3 concentration of 5%. The sensitivity of the sensor showed a small temperature dependence within a 5–35 ℃ range. In addition, no significant degradation was observed during a 7-d-long durability test. The aqueous ammonia sensor based on a printed, porous ITO layer has been explained in terms of reduction reaction at the surface and the formation of an electrical double layer (EDL).