Tridosan, a widely used antimicrobial agent, can increase colitis-associated colon tumorigenesis, and induce liver fibrosis and cancer in mice through mechanisms which may be relevant in humans. In this study, an analytical method using gas chromatography-mass spectrometry (GC-MS) and high resolution gas chromatography-high resolution mass spectrometry (HRGC-HRMS) was developed to measure dioxins and chlorinated derivatives from triclosan in the presence of active chlorine in seawater matrix. Formation yields of dioxins and chlorinated triclosans were assessed at different initial precursor concentrations under dark and UV light irradiation conditions. Results showed that triclosan was rapidly transformed to its chlorinated derivatives, i.e. tetraclosans and pentaclosans, of which the formation yields peaked after 1 h of reaction. UV light was the key factor to promote the formation of dioxins. With the same initial triclosan/active chlorine ratio, the highest yield of dioxins was observed with lower initial concentrations of triclosan under UV irradiation. Five dioxins, including 2,8-DCDD, 1,208-TICDD, 2,3,7-TrCDD, 1,2,3,8-TeCDD, and 2,3,7,8-TeCDD, were identified and quantified. 2,3,7,8-TeCDD, the most toxic dioxin, was firstly reported as the photo transformation product of triclosan in aquatic solution. Results presented here are useful for a comprehensive understanding of the fate and toxicity of triclosan in contaminated waters.