In this paper, a high performance adsorbent (hydrous manganese oxides onto acylamino and hydroxyl functionalized hydrogel) containing manganese oxide was prepared by the cross-linking polymer; the novel composite adsorbent has an excellent adsorption for Hg(II) removal. The adsorbent was characterized by scanning electron microscopy and Fourier transform infrared analysis, the hydrogel adsorbent had a typical three-dimensional network structure, and manganese oxides particles were dispersed into the channel and pores structure. The adsorption of Hg2+ in water was tested by using the adsorbent. The results show that the adsorption isotherms were fitted well with Langmuir model, and the maximum Hg2+ adsorption capacity was 0.654 mmol g−1 (131.2 mg g−1). The adsorption kinetics followed a pseudo-second-order equation, and the adsorption equilibrium can be reached in the first 120 min. The optimum pH of adsorption was determined to be 8.0. The desorption efficiency of 94% can be reached using 0.7 mol L−1 HCl as the regeneration agent. The results suggest that this material can be a promising adsorbent for Hg2+ removal in several industrial processes.