In the present study, novel magnetic adsorbents Fe3O4@catechol/PEI were fabricated simply by copolymerization of catechol and branched polyethylenimine (PEI) onto Fe3O4 nanoparticles under alkaline condition, and characterized by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and vibrating sample magnetometer (VSM). Adsorption of three different kinds of anionic dyes, methyl blue, orange G and amaranth, from aqueous solutions using Fe3O4@catechol/PEI was investigated. The adsorption results showed that the removal of anionic dyes by Fe3O4@catechol/PEI matched well with pseudo-second-order kinetic model and Langmuir isotherm model. The maximum adsorption capacities for methyl blue, orange G and amaranth were 344.8, 1923, and 146.2 mg.g(-1), respectively. Furthermore, the anionic dyes could be selectively absorbed from dye mixtures containing anionic and cationic dyes. After adsorption, Fe304@catechol/PEI could be efficiently separated from the solution under the adscititious magnetic field and could be easily regenerated. This novel magnetic adsorbent is a promising candidate for removal of anionic dyes in the wastewater.