Synthesis of bimetallic-oxide-encapsulated magnetic nanoparticles is still significantly challenging and has rarely been attempted previously, due to the effects of lattice mismatch, weak chemical interactions and variances in growth rates between different components, as well as the difficulty in process control for uniform co-deposition. In the present work, Fe-Mn bimetallic oxide (FMBO) nanoplatelet encapsulated magnetic nanoparticles (Mag-FeMn) are prepared by controlled engineering of the interparticle coupling of Fe3O4 and FMBO, with its multifunctional capabilities highlighted in terms of the potentially superior As(III) sequestration and convenient recoverability. Multiple characterization techniques are employed to examine the derived morphologies and to accurately resolve both compositionally and magnetically the hierarchical structure in detail. The synthesized magnetic composites retain highly porous structure with the main components of Fe2O3, FeOOH, Fe3O4, and Mn3O4. Mag-FeMn exhibits a quite competitive high capacity for As(III) capture (56.1 mg g(-1)), whereby As(III) oxidation coupled with synchronous sorption contributes to the improved performance. The unique heterostructure of FMBO encapsulation with an embedded magnetic core would be applicable to help with rational synthesis of other bimetallic oxide encapsulated magnetic nanoparticles, and definitely shows promise for the development of new nanotechnology enabled approaches for adsorption-based water purification.