As(III) is the most common arsenic species in copper smelter wastewater but is difficult to remove, and an oxidation to As(V) is desirable. The oxidation of As(III) to As(V) in copper smelter wastewater was studied using in situ-generated ozone, which has a high oxidation capacity and does not create secondary contaminants. With the aim to determine adequate conditions for As(III) oxidation, experiments considering pH, ozonation time, and oxygen flow rate as variables were done. Experimental tests showed that the best pH was at 2 and with a moderate oxygen flow rate. The complete oxidation of As(III) in both synthetic wastewater and real wastewater was achieved nearly after the same time of ozonation. Pseudo-first-order kinetics could model the As(III)-to-As(V) oxidation by ozone.