We report the template-free fabrication of 3D iron hydroxide hierarchical nanostructures through a simple and low-cost self-assembly process using a galvanic-cell reaction at room temperature. The existence of sodium sulfate in the reaction system is critical for formation of the iron hydroxide hierarchical nanostructures. X-ray diffraction analysis reveals that, after calcination, the iron hydroxide nanostructures can be changed into magnetic iron oxides. Relevant scanning electron microscopy and transmission electron microscopy images show that both of the flower-like nanostructures are composed of porous nanosheets. We also demonstrate that the two hierarchical nanostructures described above are negatively charged when dispersed in water. When used as adsorbents, they can selectively remove neutral dyes from wastewater with much higher capacities than those of conventional α-FeOOH and α-Fe₂O₃ nanoparticles, indicating great potential for use in water treatment.