Bifunctional mesoporous g-C3N4/Fe3O4 composites were prepared and used for the simultaneous visible-light catalysis and adsorption of arsenic from aqueous solution. The as-prepared composites were characterized by wide-angle X-ray powder diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet visible diffuse reflectance spectroscopy, a vibrating sample magnetometer, and Brunauer-Emmett-Teller surface analysis. Experimental results showed that meso-g-C3N4/Fe3O4 composites could oxidize As(III) to As(V), which was in turn effectively removed by the composites. The photocatalytic activity was significantly enhanced by the synergistic effect between g-C3N4 and Fe3O4. In addition, a reasonable visible-light catalytic oxidation mechanism of As(III) was investigated.