In this study, the removal kinetics and degradation mechanism of 1H-benzotriazole (BTA), which is widely used in anti-corrosive agents, during the UV/chlorination reaction were investigated, focusing on the UV-A/chlorination process. The UV-A/chlorination reaction showed a synergetic effect on the removal of BTA compared to UV-A photolysis or chlorination alone. The synergetic effect was due to the generation of hydroxyl radicals and reactive chlorine species. The removal of BTA during UV/chlorination followed pseudo-first-order kinetics. Faster removal of BTA was achieved when the chlorine dosage was increased. Alkaline pH favored the removal of BTA during UV-A/chlorination. The UV/chlorination of BTA achieved approximately 50% mineralization confirmed by total organic carbon within 12 h. As byproducts, formate and nitrate ions and five organic byproducts (m/z = 150.0307, 166.0252, 124.0147, 140.0071, and 154.0245) were identified by ion chromatography and LC-qTOF-MS. Based on the identified byproducts, we proposed the degradation pathway of BTA during the UV-A/chlorination reaction. Finally, toxicity test was performed during the UV-A/chlorination of BTA using a Microtox bioassay. The toxicity decreased during UV-A/chlorination of BTA, implying that the byproducts are less toxic than BTA. Our study indicates that the UV-A/chlorination process is effective not only in eliminating BTA, but also in decreasing toxicity.