Novel chrysanthemum-like 3D hierarchical magnetic γ-Fe₂O₃ and Fe₃O₄ superstructures self-assembled by nanobelts have been successfully synthesized through a simple one-pot solvothermal route without using structure-directing additives. The morphology and structure of the sample depend strongly on the reaction time. γ-Fe₂O₃ and Fe₃O₄ with superparamagnetic properties were obtained by thermal transformation of the precursor without changing the overall architectures. A possible formation mechanism of the chrysanthemum-like 3D hierarchical precursor was proposed. The BET surface area and the magnetic properties of the prepared γ-Fe₂O₃ and Fe₃O₄ were investigated. In addition, the adsorption capacity of the as-prepared magnetic iron oxide for organic pollutants was also determined by using 2,4-dichlorophenol and 2,4,6-trichlorophenol as probe pollutants.