Abiotic iron monosulfide (FeS) has attracted growing interests in dechlorinating trichloroethylene (TCE) in anoxic groundwater, but it is still unclear how biogenic FeS affects the dechlorination and thus the cytotoxity of TCE. In this work, a biogenic FeS was synthesized by Shewanella oneidensis MR-1 with addition of ferrihydrite and S 0 , and it was used for dechlorination of TCE in alkaline environment and the de-cytotoxicity was evaluated by the growth of Synechocystis sp. PCC6803. The results show that the biogenic FeS was of mackinawite, with a loose flower-like mosaic structure. The dechlorination of TCE by the biogenic FeS was accelerated by 6 times than that by abiotic FeS. TCE was dechlorinated mainly by hydrogenolysis to form dichloroethane (C 2 H 2 Cl 2 ), vinyl chloride (C 2 H 3 Cl), and finally ethylene, accompanied with transformation of both Fe 2+ to Fe 3+ and monosulfide to disulfide and polysulfide on the biogenic FeS surface. The concentration for 50% of maximal inhibition effect (EC 50 ) of TCE to Synechocystis was 486 mg/L and the inhibition to Synechocystis under the EC 50 was relieved more significantly on addition of the biogenic FeS than that of abiotic FeS. These results indicate that the biogenic FeS promoted the dechlorination and thus de-cytotoxity of TCE. Graphic abstract