In this study, a weak magnetic field (WMF), superimposed with a permanent magnet, was utilized to improve ZVI corrosion and thereby enhance As(V)/As(III) removal by ZVI at pH_ini 3.0–9.0. The experiment with real arsenic-bearing groundwater revealed that WMF could greatly improve arsenic removal by ZVI even in the presence of various cations and anions. The WMF-induced improvement in As(V)/As(III) removal by ZVI should be primarily associated with accelerated ZVI corrosion, as evidenced by the pH variation, Fe²⁺ release, and the formation of corrosion products as characterized with X-ray absorption fine structure spectroscopy. The arsenic species analysis in solution/solid phases at pH_ini 3.0 revealed that As(III) oxidation to As(V) in aqueous phase preceded its subsequent sequestration by the newly formed iron (hydr)oxides. However, both As(V) adsorption following As(III) oxidation to As(V) in solution and As(III) adsorption preceding its conversion to As(V) in solid phase were observed at pH_ini 5.0–9.0. The application of WMF accelerated the transformation of As(III) to As(V) in both aqueous and solid phases at pH_ini 5.0–9.0 and enhanced the oxidation of As(III) to As(V) in solution at pH_ini 3.0.