Cost-effective production of solar fuels requires robust and earth-abundant oxygen evolution reaction (OER) catalysts. Herein, we report that ultrathin nanoplates of cobaltmanganese layered double hydroxide (CoMn LDH) are a highly active and stable oxygen evolution catalyst. The catalyst was fabricated by a one-pot coprecipitation method at room temperature, and its turnover frequency (TOF) is more than 20 times higher than the TOFs of Co and Mn oxides and hydroxides, and 9 times higher than the TOF of a precious IrO2 catalyst. The activity of the catalyst was promoted by anodic conditioning, which was proposed to form amorphous regions and reactive Co(IV) species on the surface. The stability of the catalyst was demonstrated by continued electrolysis.