Phosphate may affect the behavior of metal(loid)s in environments, so the interaction of Sb(III) with FeS affected by phosphate was investigated in this study. Sb(III) was reacted with synthesized FeS in solution at pH 5.5, 7.0, and 9.0 with and without the addition of phosphate. The concentrations of Fe(II), Sb(III), and phosphate in the solution were monitored, and the solid phases were examined by XRD, TEM, and XPS. For the interaction of Sb(III) with FeS in the absence of phosphate, the formation of Sb₂S₃ was very important at pH 5.5 and 7.0, whereas Sb(III) adsorption was the dominant process at pH 9.0. The interaction of Sb(III) with FeS could be significantly affected by phosphate, and this impact was dependent of pH and phosphate concentration. At pH 5.5 and 7.0, the effect of phosphate varied markedly with increasing phosphate concentration. Low phosphate loadings (9.5 and 19 mg/L) had a negligible effect on the mobility of Sb(III), while high phosphate loadings (95 and 475 mg/L) notably enhanced the uptake of Sb(III) by FeS. In the case of high phosphate loadings, Fe₃(PO₄)₂‧8H₂O was formed and resulted in the dissolution of FeS, which was favorable for the formation of Sb₂S₃ and consequently the uptake of Sb(III). In the experiments at pH 9.0, the addition of phosphate only resulted in minor desorption of Sb(III) because FeS was very stable and consequently both Fe₃(PO₄)₂‧8H₂O and Sb₂S₃ were unlikely to be formed. The results of this work may help to better understand the effect of phosphate on the behavior of Sb in some anoxic environments where FeS is present.