A novel magnetic nanomaterial (Fe3O4-g-C3N4) based on Fe3O4 nanoparticles anchored on g-C3N4 was synthesized by a facile ultrasonic method and characterized by various means, including scanning electron microscopy, X-ray diffraction, vibrating sample magnetometer (VSM), Fourier transform infrared, thermogravimetric analysis, and particle size analyzer. Fe3O4-g-C3N4 was used as an magnetic adsorbent to remove Cd(II), Pb(II), and Zn(II) from aqueous solutions. Langmuir model was investigated to be the best one for fitting isothermal adsorption equilibrium data, and the corresponding adsorption capacities were predicted to be 168.93, 189.36, and 281.28 mg/g for Cd(II), Pb(II), and Zn(II), respectively. The X-ray photoelectron spectroscopy results revealed that the good adsorption ability of Fe3O4-g-C3N4 originated from the conjugation effect between Fe3O4-g-C3N4 and metal ions. All these results show the Fe(3)O(4-)g-C3N4 as one of potential adsorbents in removing Cd(II), Pb(II), and Zn(II) from wastewater.