The contamination of chromium (Cr(VI)) in groundwater threatens public health because of the industrial development being historically less supervised. To remove Cr(VI) from groundwater and other surface waters, a new FeCl3-modified magnetic biochar (MBPH550) was prepared from a waste peanut hull. The magnetic potential was to facilitate the separation of absorbent from the water phase. The removal efficiency, adsorption kinetics and isotherm models, and adsorption mechanism of Cr(VI) on MBPH550 were investigated. MBPH550 had a large specific surface of 243.23 m2/g and saturation magnetization of 5.65 emu/g. MBPH550 achieved a removal efficiency of Cr(VI) up to 92.2% +/- 2.0%. Low pH favored the adsorption of Cr(VI). At pH 6.0, the adsorption of Cr(VI) on MBPH550 fits the Elovich kinetic model and Freundlich isothermal adsorption model, with an equilibrium adsorption capacity of 6.64 mg/g. In addition to the direct functional group coordination and precipitation on the surface of MBPH550, Cr(VI) was also reduced to Cr3+, Cr(OH)3, Cr2O3, and FeCr2O4, which were all deposited on the surface of MBPH550. MBPH550 could be a widely applied efficient adsorbent for removing Cr(VI) from wastewater, groundwater, and leachate of Cr-contaminated soil.