The photochemical nitrosation of dimethylamine (DMA) has been suggested as a possible pathway for the occurrence of N-nitrosodimethylamine (NDMA) in aquatic environments. The aim of this study was to investigate the formation of NDMA from the nitrosation of DMA during nitrite (NO2⁻) photolysis in aqueous solution by varying several of the experimental parameters. NDMA was formed at neutral pH, in the presence of DMA and NO2⁻ under UV-A irradiation, and exhibited an increase in concentration with irradiation time. Increasing the concentrations of DMA and NO2⁻ led to enhanced NDMA formation. Through experiments employing OH scavenger (t-BuOH), OH was found to play an important role in the production of nitrosating agents during the photolysis of NO2⁻. The optimum pH for the formation of NDMA was around 10. This pH dependence was explained by the dual role of deprotonated DMA as a nucleophile, which enhances the nitrosation reaction between nitrosating agents and DMA, and a hydroxyl radical (OH) scavenger, which reduces the production of nitrosating agents. A higher concentration of NDMA was obtained under N2 condition due to the lower quantum yield for NDMA photolysis in the absence of oxygen. Various anions, including phosphate ions in the buffer solution, retarded the formation of NDMA, possibly due to degradation of the nitrosating agents.