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UV photolysis is an effective process to remove nitrosamines from contaminated water resources. Nitrosamines represent a class of compounds with high potential for carcinogenicity and, therefore, there are serious concerns regarding their threat to human health and their environmental toxicity. Although the photochemical parameters of parent nitrosamines and their initial reaction pathways are well understood, the fate of nitrogen-containing species and reactive nitrogen species generated from nitrosamine degradation has not yet been elucidated. In this study, we develop an elementary reaction-based kinetic model for the photolysis of N -nitrosodimethylamine (NDMA) and the photochemical transformation products. We use density functional theory quantum mechanical calculations to calculate the aqueous-phase free energies of activation and reaction to investigate the kinetics and thermodynamics properties of the elementary reactions. We generate ordinary differential equations for all species involved in the identified reactions and solve them to obtain the time-dependent concentration profiles of NDMA and the degradation products at pH 3 and pH 7. The profiles are compared to experimental results in the literature to validate our elementary reaction-based kinetic model. This is the first study to develop an elementary reaction-based kinetic model for the photochemical reaction of NDMA and reactive nitrogen species. The findings of this study have a significant impact on the active research area of nitrosative stress and advanced oxidation processes that utilize nitrogen-containing compounds such as UV/nitrate and UV/chloramine advanced oxidation processes.