Roxarsone adsorption from water using ordered mesoporous carbon CMK-3 (P_CMK-3) and CMK-3 carbon modified with thiourea (TU_CMK-3) or diclofenac (SD_CMK-3) was studied. The adsorption of roxarsone occurred at low pH. The adsorption was fast; in the case of SD_CMK-3 and TU_CMK-3 materials, the adsorption equilibrium was achieved within 2 min, whereas in the case of P_CMK-3 material, within 30 min. The adsorption capacities towards roxarsone were 130, 99, and 200 mg/g for P_CMK-3, TU_CMK-3, and SD_CMK-3, respectively. Coexistence of 20 mg/L Ca2+, Mg2+, Fe3+; Cl-, HCO3–, SO42-, NO3–, and PO43- ions did not significantly hinder the adsorption of roxarsone onto SD_CMK-3 material. The NaOH solution application enabled virtually complete roxarsone desorption from the SD_CMK-3 surface. It was found that the adsorption of roxarsone on the SD_CMK-3 material is a complex process occurring as a result of the electrostatic interaction of anionic forms of roxarsone with a positively charged surface, hydrogen bonds, and the interactions between carbon and the aromatic ring in roxarsone. Adsorption is accompanied by partial reduction of –ONO2 groups in the roxarsone molecule and partial transformation of organic As contained in the roxarsone molecule to As(V).