The UV/chlorine advanced oxidation process (AOP), which generates various highly reactive radical species, effectively degrades micropollutants and inactivates microorganisms. The ubiquitous presence of natural organic matter (NOM) in water leads to the generation of chlorinated disinfection byproducts, while the presence of bromide further induces the generation of brominated disinfection byproducts, which pose threats to ecological safety and human health. However, research has primarily focused on conventional disinfection by-products due to the complexities associated with NOM, the diversity of halogenated byproducts, and the limitations of analytical techniques. In this study, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was applied to elucidate the influence of bromide on the transformation of NOM and the generation of halogenated byproducts in the UV/chlorine AOP. The introduction of bromide in the UV/chlorine AOP facilitated the generation of hypobromous acid and bromine-containing radicals (such as Br•, BrO•,  and BrOH•−). This alteration in radical speciation led to significant transformations of CHO compounds in NOM, particularly those with higher aromaticity. The newly generated CHO compounds were mainly tannin-like and alicyclic molecules rich in carboxyl groups. A total of 222 bromine-containing compounds (CHOBr) were detected after the UV/chlorine AOP. Analysis of the precursors of these CHOBr, when considering both addition reactions (AR) and substitution reactions (SR) as the pathways, showed that these precursors had low O/C ratios or high H/C ratios. These precursors were predominantly lignin-like compounds, or highly unsaturated and phenolic compounds. The SR precursors had lower aromaticity compared to AR precursors. The majority of CHOBr compounds generated during chlorination were removed during the post-chlorination process, while most of the CHOBr compounds generated in the UV/chlorine AOP remained stable during the post-chlorination phase. This study characterizes the formation of unknown brominated by-products at the molecular level and elucidates the transformation mechanisms of their precursors. These findings highlight the importance of brominated byproducts and provide a reference for addressing critical issues in the practical application of the UV/chlorine AOP.