The degradation behavior of the pesticide Irgarol 1051 in pure (Milli-Q) and river water samples in response to photolysis and Fenton reactions was investigated and the hydroxyl radical (•OH) formation rate was determined. Irgarol photolysis was found to occur at a much slower rate in both types of water in terms of the half-life (t1/2) and degradation rate constant (k). Prolongation of photolysis did not lead to complete degradation, even after 480 min. Conversely, Irgarol was rapidly degraded in pure water at pH 3 after 12 and 16 min of irradiation via the photo-Fenton reaction (Fe2+/H2O2/UV-Vis) and photo-Fenton-like reaction (Fe3+/H2O2/UV-Vis), respectively, because of the generation of additional •OH. Irgarol was also completely degraded by the Fe2+/H2O2/UV-Vis and Fe3+/H2O2/UV-Vis reactions in river water at pH 3 after 60 min, while it disappeared after 180 min of irradiation by the Fe2+/H2O2/UV-Vis reaction at pH 5. The photodegradation rates of Irgarol in pure and river water were consistent with the •OH generation rates in both types of water. Mineralization of 96.9 and 92.9% of the Irgarol was achieved in pure water at pH 3 after 16 h of irradiation by the Fe3+/H2O2/UV-Vis and Fe2+/H2O2/UV-Vis systems, respectively. Overall, these results indicate that photo-Fenton reactions are useful for treating Irgarol-contaminated water.