Arsenic (As)-contaminated paddy soil could result in elevated levels of As in rice plants and sequentially harm human health. The Fe3O4-modified biochar (NBC-Fe) prepared by the coprecipitation method was applied in a pot experiment to investigate its effect on mobility and bioavailability of As in soil and to reduce As accumulation in rice tissues (brown rice, husks, spikelets, leaves, stems, and roots). Compared with non-application (CK), application of NBC-Fe significantly increased the cation exchange capacity (CEC), decreased As availability, and raised the As concentration of crystalline hydrous oxide-bound fraction in the soil. The addition of 0.05-1.6% (w/w) NBC-Fe significantly reduced the As concentrations in brown rice by 9.4-47.3%, which was lower than the level set by the National Food Safety Standards of China (0.2 mg/kg). The NBC-Fe treatment decreased As concentrations in iron plaque (DCB-As), and the DCB-As had the very significant correlations (P < 0.01) with the As concentrations in different rice tissues (brown rice, husks, spikelets, leaves, stems, and roots). The NBC-Fe immobilized As to decrease As availability in soil and increased the amount and thickness of iron plaque to sequester As on the surfaces of rice root. This study demonstrates that NBC-Fe is a promising soil amendment for the remediation of As-contaminated soil, therefore reducing As accumulation in rice plant and safety risks for rice consumption.