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Addressing arsenite pollution in groundwater has drawn great attention. It is attractive to pre-oxidize highly mobile As(III) to relatively low-toxic As(V) with a subsequent adsorption separation process. Herein, BiOI anchoring on γ-Fe2O3is performed to synthesize BiOI/γ-Fe2O3core–shell nanoparticles for efficient removal of As(III) via a simultaneous photocatalytic oxidization–adsorption process. The physical and chemical structures of BiOI/γ-Fe2O3are investigated by transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction measurements. The photoluminescence and electron spin resonance (ESR) characterization were employed to ascertain the possible reaction mechanism of visible-light-driven photocatalytic oxidation of As(III). Such BiOI/γ-Fe2O3delivers a superior As(III) removal capability under visible light irradiation with an arsenic removal efficiency of 99.8% within 180 min, higher than those of BiOCl/γ-Fe2O3(81.7%) and BiOBr/γ-Fe2O3(98.9%). The optimal BiOI/γ-Fe2O3(molar ratio of 2:1) is obtained by rationally adjusting the molar ratio of BiOI to γ-Fe2O3. The as-synthesized BiOI/γ-Fe2O3performs well in a wide pH range of 2–8. Only coexisting PO43−anions have a significant effect on the As(III) removal. The free radical trapping experiment and ESR results demonstrate that the ⋅O2−and h+are the main active substances for the photocatalytic oxidation of As(III) on BiOI/γ-Fe2O3. This work not only gives a novel magnetic core–shell nanoparticle photocatalyst for efficient photocatalytic oxidation and adsorption of As(III) but also offers a new strategy to rationally design BiOX for its related practical applications.