A novel biochar was successfully prepared through modification of birnessite to improve its capability to adsorb As in water and soil. The saturated adsorption capabilities of birnessite-loaded biochar (BRB) for As(III) and As(V) were as large as 3621 and 2381 mg kg−1 (calculated by Langmuir isotherm model), much higher than for the corresponding non-loaded biochar (no sorption of As). In comparison with control, after 6 weeks’ incubation in contaminated soil, BRB showed potential for reducing As concentration under flooded condition in pore water, respectively. These results suggested that BRB could be used as an effective sorbent for simultaneous immobilization of heavy metals, especially As in environmental and agricultural systems.