Photochemically generated reactive species are involved in photodegradation of dissolved organic pollutants in natural waters. However, there is a dearth of empirical evidence, from each batch of water samples collected, to predict the influence of interactive reactions among several photogenerated reactive species on their variety and fates in natural waters. Concerted photogeneration and measurement of hydroxyl ((OH)-O-center dot), nitric oxide (NO center dot) and superoxide (O-2(center dot-)) radicals were carried out on water samples obtained during two consecutive summers in 2016 and 2017 from the Seto Inland Sea, Japan. Photogeneration rates of (OH)-O-center dot are (6.98-35.27) x 10(-12) M s(-1), and those of NO center dot are (1.20-58.25) x 10(-12) Ms-1. Compared with these generation rates, that for O-2(center dot-)((4.54-18.20) x 10(-10) Ms-1) was the highest, which suggests that O-2(center dot-) is a very important photochemically generated reactive species in coastal seawater. The average steady-state concentrations of the three reactive species are (OH)-O-center dot, 7.23 x 10(-18) M; O-2(center dot-), 3.79 x 10(-12) M; and NO center dot, 1.39 x 10(-10) M. Estimated mutual consumption or sink percentages via interactive reactions between O-2(center dot-) and NO center dot radicals are five to nine orders of magnitude higher than any other radical pair considered in this study. Hence, we predict that the reaction between photochemical O-2(center dot-) and NO center dot could dominate to form ONOO-, a powerful oxidant and nitrating agent, in the coastal marine environment.