As　most　of　the　fossil　carbon　on　earth　is　stored　in　an　oxidized　state　as　carbonate　minerals,　exploration　of　metal　carbonate　catalyst　for　selective　CO2　reduction　can　open　appealing　access　to　boost　the　CO2　capture　and　storage.　However,　carbonate　is　commonly　regarded　as　a　poor　matrix　to　host　photocatalytic　active　species,　and　current　literature　has　few　reports　of　carbonate-based　photocatalytic　material.　Herein,　a　hierarchically　porous　catalyst　that　features　≈5 nm　deficient　NiCO3　nanoparticles　embedded　in　sponge-like　high-magnesium　calcite　is　disclosed.　In　the　photocatalytic　CO2　reduction　reaction,　the　as-prepared　catalyst　of　low　Ni　content　attains　a　high　CO　production　rate　of　10　565　µmol　g−1　h−1　and　a　high　selectivity　of　94%　relative　to　H2　evolution,　a　performance　that　surpasses　many　other　state-of-the-art　nickel-based　catalysts.　Experiments　and　theoretical　calculations　reveal　that　the　carbonate　vacancy　of　NiCO3　strengthens　the　adsorption　and　activation　of　CO2　more　significantly　than　the　corresponding　oxygen　vacancy　of　NiO.　The　inert　CO2　molecule　becomes　highly　deformed　on　the　surface　of　NiCO3　which　can　be　readily　activated　to　the　key　intermediate　CO2·−　for　the　photoreduction　reaction.　The　present　findings　add　to　the　existing　knowledge　of　advanced　catalysis　using　defect　materials　and　demonstrate　an　intriguing　and　rare　case　of　highly　performing　carbonate-based　catalyst.　©　2022　Wiley-VCH　GmbH.