Dimensionally stable anodes (DSAs) are regarded to be optimized electrodes for electrochlorination owing to their excellent electrocatalytic activity for the chlorine evolution reaction (CER). However, in dilute chloride solutions, DSAs preferentially produce oxygen rather than chlorine because of their low overpotential for oxygen evolution reaction (OER). Considering the frequent use of electrochlorination in dilute conditions, the poor efficiency of DSAs severely limits their environmental and industrial applications. Therefore, the aim of this study is to improve the CER efficiency of IrO2 in dilute chloride solutions with the addition of Fe2O3 as a co-catalyst which has a slow reaction rate of OER. In a dilute chloride solution of 1 mM NaCl, Ir0.11Fe0.25O0.64 showed a far higher current efficiency for CER (28%) than that of IrO2 (3%). Ir0.11Fe0.25O0.64 also exhibited better current efficiency in NaCl solution of various concentrations (1 mM to 2 M) than that of IrO2. This is attributed to the synergistic effect of Fe2O3 (slow OER rate) and IrO2 (fast CER rate). Moreover, the long-term stability of Ir0.11Fe0.25O0.64 was demonstrated with tap water electrolysis for 50 days. These results suggest that Ir0.11Fe0.25O0.64 has great potential to expand the scope of application of the electrochlorination system, particularly in dilute solutions.