Fe3O4–biochar from sugarcane bagasse was used as a biosorbent to remove Safranin O (SO) dye in an aqueous solution. The surface characteristics of Fe3O4–biochar materials were studied by SEM/EDX method and point of zero charge (pHpzc). Batch adsorption experiments were carried out to evaluate the influence of solution pH, Fe3O4–biochar dosage, adsorption time, and initial SO concentration on the adsorption of SO by Fe3O4–biochar. The results showed that pH=6, Fe3O4–biochar dosage of 3 g in 50 mL SO sulution, adsorption time of 120 minutes, and initial SO concentration of 50 mg/L were optimum conditions for SO adsorption process. The Langmuir isotherm adsorption models described well the SO adsorption process at room temperature, with the regression coefficient R2 of 0.94. The maximum adsorption capacity of SO calculated by the Langmuir model was 12.18 mg/g. The study demonstrated that the Fe3O4–biochar can be easily separated from the solution by using an external magnet and can be effectively used to remove SO cationic dye in an aqueous solution.