Blue carbon ecosystems provide a wide range of ecosystem services, are critical in maintaining marine biodiversity, and may potentially serve as sites of economically viable carbon dioxide removal through enhanced organic carbon storage. Here we show that restoring two key blue carbon ecosystems — mangroves and seagrasses — will also foster high rates of permanent atmospheric carbon dioxide removal (potentially up to ~10 tCO2 ha-1 year-1) by driving ocean alkalinity enhancement.