The effect of oxalate on the degradation of ranitidine was investigated in the titanium dioxide (TiO2)/hydrogen peroxide (H2O2) system. The degradation of ranitidine proceeded more rapidly, with a 5.7-fold increase in the presence of oxalate. The oxalate-enhanced degradation is attributed to the higher production of hydroperoxyl radicals (HO2 ●) through the complexation of oxalate on the >TiIII (>TiIII(oxalate)n (2n−3)−), generated from inner-sphere electron transfer in the peroxo metal complexes (>TiIV−OOH). This was confirmed by measuring the time-dependent concentrations of HO2 ●, hydroxyl radicals (●OH), and H2O2 both with and without oxalate. The results of reactive oxygen species (ROS)-quenching experiments show that both HO2 ● and singlet oxygen (1O2) are primary oxidants in the TiO2/H2O2/oxalate system. The pseudo-first-order degradation rate constants (k) for ranitidine were higher with increasing TiO2 dosage (up to [TiO2] = 1.0 g/L) and H2O2 concentration but were optimal at [oxalate] = 1 mM. Although other chelating agents, such as citrate, acetate, and malonate, also showed a positive effect on the degradation of ranitidine in the TiO2/H2O2 system, oxalate exhibited the best performance. In addition, the extent of positive effect of oxalate in the TiO2/H2O2 system was much higher than that in the tungsten oxide (WO3)/H2O2 system.