<p>Eutrophication leads to massive algal proliferation. During algal blooms, cyanobacteria often serve as the dominant species, while green algae are frequently the subdominant species. Algal organic matter can become a potential source for the formation of halonitromethanes (HNMs). During ultraviolet/chlorine treatment, bromide ions (Br⁻) promote the formation of brominated halonitromethanes (Br-HNMs), which exhibit greater toxicity compared to chlorinated halonitromethanes (Cl-HNMs). While the formation of Br-HNMs from cyanobacteria has been documented, research on how green algae contribute during UV/chlorine disinfection in the presence of Br⁻ remains limited. Therefore, <i>Chlorella vulgaris</i>, a widely distributed green alga, was selected as a model precursor to investigate the formation patterns and toxicity of Br-HNMs derived from its intracellular organic matter (IOM) during UV/chlorine disinfection. Bromonitromethane (BNM) and bromodichloronitromethane (BDCNM) were observed to form from the IOM of <i>Chlorella vulgaris</i>, with their concentrations rising initially and then falling as Br⁻ concentration and reaction time increased. Additionally, higher free chlorine concentration, UV intensity, and IOM concentration promoted Br-HNMs formation (i.e., BNM and BDCNM), whereas an increase in pH inhibited their formation. Potential pathways for the formation of Br-HNMs were deduced based on the experimental results. Moreover, Br-HNMs formation patterns from the IOM of <i>Chlorella vulgaris</i> in actual water samples closely resemble the results in simulated waters. This study elucidates the risks associated with Br-HNMs formation from the IOM of <i>Chlorella vulgaris</i> during UV/chlorine disinfection. These findings provide theoretical and technical support for optimizing water treatment processes and controlling Br-HNMs formation at water treatment facilities.</p>