AbstractElectron density modulation is of great importance in an attempt to achieve highly active electrocatalysts for the oxygen evolution reaction (OER). Here, the successful construction of CuO@CoOOH p‐n heterojunction (i.e., p‐type CuO and n‐type CoOOH) nanoarray electrocatalyst through an in situ anodic oxidation of CuO@CoS_x on copper foam is reported. The p‐n heterojunction can remarkably modify the electronic properties of the space‐charge region and facilitate the electron transfer. Moreover, in situ Raman study reveals the generation of SO₄²⁻ from CoS_x oxidation, and electron cloud density distribution and density functional theory calculation suggest that surface‐adsorbed SO₄²⁻ can facilitate the OER process by enhancing the adsorption of OH⁻. The positively charged CoOOH in the space‐charge region can significantly enhance the OER activity. As a result, the CuO@CoOOH p‐n heterojunction shows significantly enhanced OER performance with a low overpotential of 186 mV to afford a current density of 10 mA cm⁻². The successful preparation of a large scale (14 × 25 cm²) sample demonstrates the possibility of promoting the catalyst to industrial‐scale production. This study offers new insights into the design and fabrication of non‐noble metal‐based p‐n heterojunction electrocatalysts as effective catalytic materials for energy storage and conversion.