Three-dimensional (3D) urchin-like Fe-Mn binary oxide (UFMO) was designed and successfully prepared by a facile one-pot template-free method. The as-synthesized UFMO, having a high specific surface area of 142 m² g^–1, consisted of an amorphous Fe-Mn binary oxide (FeMn_xO_y) core and a well-aligned α-FeOOH nanorod shell growing radially outward. Based on the systematic analysis of products formed at different synthetic stages, a plausible two-stage growth process, fast nucleation followed by slow crystallization, was proposed to illustrate the formation of the 3D hierarchical nanostructure. The internal structure of UFMO can be manipulated by tuning the molar ratio of Fe/Mn. Owing to its 3D hierarchical morphology and heterogeneous surface chemical composition, UFMO can serve as a promising adsorbent for the removal of heavy metals from water and a potential electrode material for supercapacitor in the field of energy storage. UFMO was able to reduce the concentration of both As(III) and Cd(II) from 100 to 1 ppb at neutral pH with a dosage of 0.1 g L^–1. The maximum adsorption capacities for As(III) and Cd(II) are 147.2 and 79.06 mg g^–1, respectively. Furthermore, the fast gravimetrical separation from solution and good regenerability of UFMO implies a green, economic, and sustainable approach for large-scale water purification applications. UFMO also exhibited a high specific capacitance of 158 F g^–1 at a current density of 0.5 A g^–1 (20 A m^–2) and good reversibility with a cycling efficiency of 90% after 1000 cycles in the electrochemical performance tests. In general, UFMO has great potentials in water treatment and energy storage applications, owing to its high performance, low cost, and environmentally benign nature.