In summary, we have elucidated the ability of oxidized phosphorene nanoflakes (PhosO) for the simultaneous removal of methylarsenicals from contaminated water sources. The PhosO nanoflakes form inner-sphere surface complexes with methylarsenicals under water conditions, even with enhanced adsorption stability compared to intrinsic phosphorene and without the competition of water molecules for adsorption sites. The inner-sphere surface adsorption of trivalent methylarsenicals is driven by electrostatic forces and contributed from charge-transfer (orbital) stabilization. While surface complexation of pentavalent methylarsenicals occurs by a balanced contribution of orbital and long-range driving forces, allowing relative higher adsorption energies than trivalent pollutants; anionic contaminants also show a high stabilization via extra polarization effects. Furthermore, recovery of nanoadsorbent after uptake turns convenient via inexpensive alkaline post-treatments. Therefore, phosphorene oxide could serve as a remarkable basis material for future technologies related to adsorption, removal, and/or control of water-soluble methylarsenicals.