The adsorption of roxarsone by iron (hydr)oxide-modified multiwalled carbon nanotubes (MWCNTs) from aqueous solution and its mechanism were investigated. The amount of roxarsone adsorbed by modified MWCNTs is higher than MWCNTs at the same condition. The results show that the sorption isotherms are nonlinear, and can be well fitted according to the Freundlich and Polanyi–Manes models. The adsorption kinetics could be well described by pseudo-second-order model. Thermodynamic analysis shows that the adsorption of roxarsone on the adsorbents is an exothermic and spontaneous process. Desorption process had no obvious desorption hysteresis phenomenon. The FT-IR and XPS analysis confirmed the loading of iron ions on the MWCNTs, and revealed the involvement of carboxyl groups and iron ions on the adsorption of roxarsone.