The effect of As­(III) on the photocatalytic degradation of phenolic pollutants such as 4-chlorophenol (4-CP) and bisphenol A (BPA) in a suspension of platinized TiO2 (Pt/TiO2) was investigated. In the presence of As­(III), the photocatalytic degradation of 4-CP and BPA was significantly enhanced, and the simultaneous oxidation of As­(III) to As­(V) was also achieved. This positive effect of As­(III) on the degradation of phenolic pollutants is attributed to the adsorption of As­(V) (generated from As­(III) oxidation) on the surface of Pt/TiO2, which facilitates the production of free OH radicals (•OHf) that are more reactive than surface-bound OH radicals (•OHs) toward phenolic pollutants. The generation of •OHf was indirectly verified by using coumarin as an OH radical trapper and comparing the yields of coumarinOH adduct (i.e., 7-hydroxycoumarin) formed in the absence and presence of As­(V). In repeated cycles of 4-CP degradation, the degradation efficiency of 4-CP gradually decreased in the absence of As­(III), whereas it was mostly maintained in the presence of As­(III), which was either initially present or repeatedly injected at the beginning of each cycle. The positive effect of As­(III) on 4-CP degradation was observed over a wide range of As­(III) concentrations (up to mM levels) with Pt/TiO2. However, a high concentration of As­(III) (hundreds of μM) inhibited the degradation of 4-CP with bare TiO2. Therefore, Pt/TiO2 can be proposed as a practical photocatalyst for the simultaneous oxidation of phenolic pollutants and As­(III) in industrial wastewaters.