Fe3O4/activated carbon (Fe3O4/AC) composites were synthesized from cyanobacteria for the first time through a simple immersion and calcination process. The composite was prepared by using cyanobacteria, and this preparation provided environmental benefits. The as-synthesized Fe3O4/AC composite was then characterized and then used to remove methylene blue (MB) from aqueous solution under various experimental conditions. Results demonstrated that MB is physically adsorbed onto Fe3O4/AC composites but chemically adsorbed on monolayer and heterogeneous surfaces. The maximum adsorption capacity (qm) determined from the Langmuir isotherm is 207.90 mg g(-1). This high adsorption capacity may be attributed to small particles, increased surface area, and functional groups on the surface of the composite. The adsorption mechanism was also discussed. These results indicated that Fe3O4/AC composite is a promising and competitive absorbent with abundant source, easy separation process, and environmentally friendly characteristics for wastewater treatment. (C) 2015 Elsevier B.V. All rights reserved.