Chromium(VI) is a contaminant of priority concern, which can be reduced and immobilized by transformation of toxic Cr(VI) to non-toxic chromite (Cr(III)) in water and soil environments. Siderite (FeCO₃), an Fe(II)-containing mineral which occurs in many anoxic soils, sediments and groundwater systems, can be extremely redox reactive, thus making it a potentially important phase governing the fate and transport of Cr(VI) in many subsurface environments. Here we investigate the sorption and reduction of Cr(VI) on siderite, as well as speciation of Cr(VI)-reacted siderite as a function of varying pH (4–10) and initial Cr(VI) concentrations (0.5–10 mM) under strictly O₂-free experimental conditions. The maximum Cr(VI) removal capacity by siderite was 81 mg g^-1 at pH 5, which decreased at higher pH values. Notably, up to 97% and 91% of total aqueous chromium, at all initial added aqueous Cr(VI) concentrations (0.5–10 mM), was sorbed/reduced to Cr(III) at pH 4 and 5, respectively, indicating a strong Cr(VI) reducing ability of siderite. At pH 6–10, over 90% of aqueous chromium was detected as Cr(III) only at initial Cr(VI) concentration ranging from 0.5–1.5 mM. Chromium K-edge X-ray absorption near edge structure (XANES) spectroscopy confirmed the complete reduction of Cr(VI) to Cr(III) after equilibration of siderite with low and high initial Cr(VI) concentrations (2 and 10 mM, respectively) at pH 5, 7 and 9. In addition, Fe K-edge XANES and extended X-ray absorption fine structure (EXAFS) spectra of solid-phase samples showed that 74–78% and 78–89% of Fe remained as siderite, respectively, after equilibration with Cr(VI) at pH 5, 7 and 9, whereas only 24% remained for the low Cr(VI) treatment at pH 5. Overall, this study suggests that under anoxic aqueous conditions, siderite can immobilize and detoxify Cr(VI), with the extent of these coupled sorption and redox reactions being controlled by initial Cr(VI) concentrations and pH.