Arsenic pollution has drawn wide attention, owing to its severe carcinogenicity, strong poisonousness, and high mobility in groundwater. Herein, a magnetic heterojunction nanocomposite of TiO2/polythiophene/gamma-Fe2O3 (TiO2/PTh/gamma-Fe2O3) was fabricated by one-pot oxidation polymerization as a novel photocatalyst for removing trace As(III) from aqueous solution by a visible-lightdriven photocatalytic oxidation-adsorption coupling process. The nanocomposite showed excellent stability to adsorb As(III) in a low-pH region (2-6). Under dark conditions, approximately 46.5% of As(III) was adsorbed by the as-synthesized nanocomposite, and the kinetic data indicated the dominating chemical adsorption. However, under visible light illumination, more than 99.1% of As(III) was transformed into As(V) and simultaneously 90.1% of produced As(V) was adsorbed by the nanocomposite. Furthermore, the photogenerated holes (h(+)) and superoxide free radicals (center dot O-2(-)) were the main reactive substances on the TiO2/PTh/gamma-Fe2O3 nanoparticle for visible-light-driven photocatalytic oxidation of As(III). The novel magnetic TiO2/PTh/gamma-Fe2O3 heterojunction nanocomposite may have potential application for arsenic removal in the aquatic environment.