<p>Iron(III) (oxyhydr)oxides are the major sink for PO₄³⁻ in freshwater sediments, helping to control the onset of eutrophic conditions when PO₄³⁻ is in excess; however, this sink is transient as ferrous iron and PO₄³⁻ are typically released when sediments become anoxic, triggering a self-sustaining reversible cycle as sediments become an internal source of phosphate. Ferrous iron can then react with dissolved sulfides to form iron sulfide minerals such as mackinawite (FeS), whose interactions with PO₄³⁻ remain largely unexplored. We investigate the role of FeS in the sequestration of PO₄³⁻ under anoxic conditions, with or without natural organic carbon (OC). We show that Fes has a high affinity for PO₄³⁻, with a maximum sorption capacity comparable to moderately crystalline FeO_x minerals. The presence of sorbed OC decreases the FeS affinity toward the PO₄³⁻ by ~ 75%, which is attributed to the lower surface area of FeS-OC compared to FeS, electrostatic repulsion, and the reduced accessibility to reactive Fe(II) sites. Moreover, we show that upon exposure of FeS to oxic conditions, PO₄³⁻ is efficiently recaptured by the FeS oxidation products (FeOx), hence highlighting a parallel self-sustaining reversible cycle resulting in the permanent sequestration of a small fraction of PO₄³⁻. Overall, these findings highlight FeS as a potentially important phosphate sink in anoxic freshwater sediments.</p>