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The chlorine evolution reaction (CER) and oxygen evolution reaction (OER) occur simultaneously due to the low difference (0.13 V) in their standard potentials. RuO2 is the state‐of‐art electrocatalyst used for both OER and CER. The activity and selectivity of different RuO2 low‐index facets, namely (100), (110), (111), (001) and (101), are investigated through ab‐initio density functional theory (DFT) based calculations. The selectivity of different facets is explored in a mixed OER‐CER region by combining Pourbaix diagrams and linear scaling relationships. The difference in limiting overpotential of OER and CER is identified as the selectivity descriptor (SDCER). The most CER‐ and OER‐selective facets are found to be (101) (SDCER=0.39 V) and (001) (SDCER=0.14 V), respectively. The understanding of facet dependent CER selectivity in RuO2 can be extended as a design strategy to modulate OER and CER activity and selectivity as per design requirements. [ABSTRACT FROM AUTHOR]