This study reports the synthesis of cobalt oxyhydroxide (CoOOH) with a high-spin state of Co³⁺, which exhibits significantly improved oxygen evolution reaction (OER) activity compared to the conventional low-spin state CoOOH. The high-spin state Co³⁺ was achieved by introducing coordinatively unsaturated Co atoms, which alter the electronic configuration and enhance electron transfer ability. The high-spin state Co³⁺ in CoOOH exhibits faster electron transfer through apex-to-apex e₉* orbitals, leading to a lower overpotential of 226 mV at 10 mA cm⁻², compared to 374 mV for the low-spin state CoOOH. This improvement is attributed to the increased number of electronic states near the Fermi level, which facilitates faster electron transfer during the OER process. The high-spin state CoOOH also demonstrates excellent structural and catalytic stability, maintaining its performance over 200 hours of operation. The findings highlight the critical role of the spin state of Co³⁺ in determining the OER activity of CoOOH-based electrocatalysts, providing a new strategy for designing highly efficient electrocatalysts for water splitting. The study also demonstrates the potential of using high-spin state Co³⁺ in other oxide-based electrocatalysts.This study reports the synthesis of cobalt oxyhydroxide (CoOOH) with a high-spin state of Co³⁺, which exhibits significantly improved oxygen evolution reaction (OER) activity compared to the conventional low-spin state CoOOH. The high-spin state Co³⁺ was achieved by introducing coordinatively unsaturated Co atoms, which alter the electronic configuration and enhance electron transfer ability. The high-spin state Co³⁺ in CoOOH exhibits faster electron transfer through apex-to-apex e₉* orbitals, leading to a lower overpotential of 226 mV at 10 mA cm⁻², compared to 374 mV for the low-spin state CoOOH. This improvement is attributed to the increased number of electronic states near the Fermi level, which facilitates faster electron transfer during the OER process. The high-spin state CoOOH also demonstrates excellent structural and catalytic stability, maintaining its performance over 200 hours of operation. The findings highlight the critical role of the spin state of Co³⁺ in determining the OER activity of CoOOH-based electrocatalysts, providing a new strategy for designing highly efficient electrocatalysts for water splitting. The study also demonstrates the potential of using high-spin state Co³⁺ in other oxide-based electrocatalysts.