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Tetrabromobisphenol S (TBBPS) is a widely used brominated flame retardant that has attracted environmental concern due to its abundant presence in water. The objective of this study is to systematically analyze the direct photolysis and degradation mechanisms of TBBPS in two different dissociation forms in water, as well as to evaluate their toxicological effects induced by •OH, 1O 2 , and •NO 2 radicals. The degradation mechanism of TBBPS is investigated with density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, and the toxicity of the degradation products is assessed through toxicological studies. The results of the study indicate that the OH-addition and H-abstraction reactions are favorable pathways for •OH-induced TBBPS degradation. The H-abstraction reaction of TBBPS0 with •OH was more favorable than the •OH addition reaction. However, in the degradation of TBBPS−, the •OH addition reaction was favored over the H-abstraction reaction. Additionally, the indirect photolysis of TBBPS by 1O 2 and •NO 2 in water was found to be easier for TBBPS− compared to TBBPS0, with degradation mechanisms involving Br-substitution and NO 2 -addition reactions. The higher E a values calculated indicate that the degradation of TBBPS by 1O 2 and •NO 2 in water has been a secondary reaction. The direct photolysis reaction pathway of TBBPS in water has involved the cleavage of the S1–C7 and S1–C16 bonds. For TBBPS0 in the S 1 /T 1 states, the primary reaction pathway is the cleavage of the S1–C16 bond, while for TBBPS−, the primary reaction pathway is the cleavage of the S1–C7 bond. Furthermore, the computational toxicology results indicate a slight increase in the toxicity levels of most products, highlighting the significance of investigating the degradation byproducts of TBBPS in greater detail. [Display omitted] • The cleavage of S1–C16 bond is the primary reaction path of TBBPS0 in S 0 /T 1 state. • The H-abstraction reaction of.•OH is easier to occur than the OH-addition reaction • 1O 2 and.•NO 2 plays a secondary role in the photodegradation of TBBPS • Most of the TBBPS degradation products are at a harmful level. [ABSTRACT FROM AUTHOR]