Naproxen is one of the most commonly used pharmaceuticals in the world. It was measured at different concentration levels in environmental water. Since naproxen can be exposed to UV light during water and wastewater treatment, an investigation was conducted to identify photolysis rate and transformation product formation during photolysis in the presence of photosensitizers. Different experiments were carried out to identify the effect of dissolved organic matter and NO3- to photolysis reactions of naproxen. Naproxen was measured and transformation products were detected using liquid chromatograph coupled with triple quadrupole tandem mass spectrometer. MassBank libraries were used to propose transformation product molecular structures. During direct photolysis experiments pseudo first order degradation kinetic constant k(obs) was found as 0.121min(-1). k(obs) decreased to 0.068min(-1) during indirect photolysis experiments. While presence of dissolved organic matter decreases photolysis rate, NO3- accelerates the degradation of naproxen. Benzyl succinic acid and caffeic acid were proposed as potential transformation products of naproxen along with an unidentified transformation product having 177m/z. All potential transformation products of naproxen proposed in this study were identified for the first time in the literature.