The rational design of metal–organic frameworks with tailored components and structural features is crucial for achieving the desired functions and expanding the emerging applications. Herein, water-stable bimetallic Fe/Mg metal–organic frameworks (Fe/Mg-MIL-88B) have been successfully fabricated through a facile and effective one-step strategy to access the exceptional arsenic decontamination. Notably, the obtained bimetallic Fe/Mg-MIL-88B architectures are demonstrated to possess a well-defined spindle-like morphology. Interestingly, the Fe/Mg molar ratios within the resultant Fe/Mg-MIL-88B frameworks can be flexibly modulated on demand, leading to the variation of the structural features associated with length/diameter ratio and unit cell parameters along with surface areas. Thanks to the unique structural and compositional merits as well as the synergetic contribution from two active metal centres, the fabricated Fe/Mg-MIL-88B nanospindles exhibit enhanced decontaminant performance toward arsenate in terms of ultrafast sorption kinetics and high sorption capacities, compared to the monometallic Fe-MIL-88B. Impressively, an extraordinary arsenate uptake capacity up to 303.6 mg/g is achieved in the optimized Fe/Mg-MIL-88B with Fe/Mg feeding ratio of 0.5, which is substantially superior to most of the reported arsenic absorbents. More significantly, these Fe/Mg-MIL-88B absorbents possess exceptional regenerative ability and stability during multiple sorption/desorption cycles, as reflected by the negligible drop in arsenic removal efficiency and excellent maintenance of the crystalline structure and morphology integrity. All these satisfactory results prefigure that the designed bimetallic Fe/Mg-MIL-88B absorbents hold a great promise for the in-depth purification of arsenic-contaminated water.