This database is a compilation of published and unpublished data used to estimate (i) the organic carbon biomass stocks, (ii) the soil organic carbon stock, and (iii) the rates of organic carbon sequestration to the soil associated to Spanish and Portuguese seagrass meadows and salt marshes. The database is presented in five .csv files. Specific .csv files content: 1) General Information: Site ID: ID of the location where the samples were collected Locality: Locality where the samples were collected Region: Spanish or Portuguese region where the samples were collected Coast: Spanish or Portuguese coast where the samples were collected Ecosystem: Type of blue carbon ecosystem Predominant Species: Predominant species at the sampling location Tidal Range: Tidal range at the sampling location. Intertidal and subtidal for seagrass meadows and Low, Medium, High and Microtidal for Salt Marshes Sampling Year (yr): Year in which the sample was collected Latitude (decimal degrees, WGS84): Latitude of sampling station. Longitude (decimal degrees, WGS84): Longitude of sampling station. Water Depth (m): Water column depth at sampling station Reference: Publications where the data has been used Researcher: Name of the person responsible for the data. Contact: Email contact of person responsible for the data. 2) Biomass: Site ID: ID of the location where the samples were collected Sample ID: ID of the sample Ecosystem: Type of blue carbon ecosystem Species: Predominant species at the sampling location Month: Month of the year in which the sample was collected Abovegr DW (km m-2): Aboveground biomass dry weight Abovegr sd (km m-2): Standard deviation of the aboveground biomass dry weight Abovegr (n): Number of samples of the aboveground biomass dry weight Belowgr DW (km m-2): Belowground biomass dry weight Belowgr sd (km m-2): Standard deviation of the belowground biomass dry weight Belowgr (n): Number of subsamples of the belowground biomass dry weight 3) Soil: Site ID: ID of the location where the samples were collected Core ID: ID of the soil core Ecosystem: Type of blue carbon ecosystem Dominant Species: Predominant species at the sampling location Tidal Range (m): Tidal range at the sampling location. Intertidal and subtidal for seagrass meadows and Low, Medium, High and Microtidal for Salt Marshes Year of sampling (yr): Year in which the sample was collected Core Compression (%): Percentage of compression due to core extraction Min depth (compacted, cm): Start depth of the sediment interval. Uncorrected for compression. Max depth (compacted, cm): End depth of the sediment interval. Uncorrected for compression. Dry Weight (g dw): Mass of dried sediment Dry bulk density (g dw cm-3): Mass of dried sediment to the total volume Organic matter (% dw): Percentage of organic matter in the sediment sample (based on dry weight) Organic carbon (% dw): Percentage of organic carbon in the sediment sample (based on dry weight) Estimated Age (years from sampling): Estimated age of the sediment Raw dates: Dates used to estimate age of the sediment Dating method: Dating method 4) OC Plant: Site ID: ID of the location where the samples were collected Sample ID: ID of the sample Ecosystem: Type of blue carbon ecosystem Species: Predominant species at the sampling location Tissue: Tissues used to estimate the carbon content TOC (%): Percentage of carbon in the plant sample (based on dry weight) TOC sd: Standard deviation of the percentage of carbon in the plant sample (based on dry weight) 5) Published: published data from the studied categories (Posidonia oceanica, Zostera marina, Zostera noltii, Cymodocea nodosa, and Low, Medium, High and Microtidal salt marshes) from outside Spain and Portugal Ecosystem: Type of blue carbon ecosystem Tidal Range: Tidal range at the sampling location. Intertidal and subtidal for seagrass meadows and Low, Medium, High and Microtidal for Salt Marshes Genus: Predominant genus Species: Predominant species at the sampling location Seagrass type: Seagrass type following the classification of Kilminster et al. 2015 (https://doi.org/10.1016/j.scitotenv.2015.04.061): Persistent, classification and opportunistic. Latitude (decimal degrees, WGS84): Latitude of sampling station. Longitude (decimal degrees, WGS84): Longitude of sampling station. Abovegr DW (kg dw m-2): Aboveground biomass dry weight Abovegr OC stock (kg dw OC m-2): Organic carbon stock in the aboveground biomass SE Abovegr OC stock (kg dw OC m-2): Standard error of the aboveground biomass organic carbon stock SD Abovegr OC stock (kg dw OC m-2): Standard deviation of the aboveground biomass organic carbon stock Abovegr OC stock n: Number of subsamples of the aboveground biomass organic carbon stocks Soil Stock 1m (kg dw OC m-2): Soil organic carbon stocks at 1m depth SE Soil Stock 1m (kg dw OC m-2): Standard error of the soil organic carbon stocks at 1m depth SD Soil Stock 1m (kg dw OC m-2): Standard deviation of the soil organic carbon stocks at 1m depth Soil Stock (n): Number of cores used to estimate the soil organic carbon stocks at 1m depth OC sequestration rate (kg dw OC m-2 yr-1): Average organic carbon sequestration rates OC sequestration rate SE (kg dw OC m-2 yr-1): Standard error of the average organic carbon sequestration rates Time frame (years): Time frame used to estimate the average organic carbon sequestration rates OC sequestration rate last 100 years (kg dw OC m-2 yr-1): Average organic carbon sequestration rates in the last 100 years OC sequestration rate last 100 SE (kg dw OC m-2 yr-1): Standard error of the average organic carbon sequestration rates in the last 100 years OC sequestration rate last 100 (n): Number of cores used to estimate the average organic carbon sequestration rates in the last 100 years Reference: DOI of the data origin