In this paper, a magnetic adsorbent of iron oxide (Fe₂O₃) with a chestnut-like amorphous-core/γ-phase-shell hierarchical nanostructure (CAHN) has been synthesized in a rationally designed chemical reaction system, where the decomposition rate of Fe(CO)₅ is controlled by the CO generated from the decomposition of N,N-dimethylformamide under the assistance of the hydrolysis of SnCl₂·2H₂O. By utilizing the current chemistry equilibrium dynamics, it is possible to kinetically modulate the nucleation and subsequent growth of Fe₂O₃ to form chestnut-like hierarchical nanostructures, in which single crystalline γ-Fe₂O₃ nanorods, with a diameter of 20 nm and a length of 300 nm, radially grow from the surfaces of amorphous and porous Fe₂O₃ sub-microspheres. The detailed possible formation mechanism of the Fe₂O₃ CAHNs is proposed according to the experimental results. The as-obtained Fe₂O₃ CAHNs show a strong adsorption capability for As(V), with a maximum adsorption capacity of 137.5 mg g⁻¹, because of both the specific surface area, which can be as large as 143.12 m² g⁻¹, and the heterogeneous surface properties. Furthermore, their ferromagnetic properties make them easy to separate from water by magnetic separation. The adsorption process obeys the Freundlich isotherm model well, but not the Langmuir model, suggesting that a multilayered adsorption occurs on the surface of the Fe₂O₃ CAHNs. Our work may shed light on the design and preparation of high performance 3D hierarchically nanostructured adsorbents.