The exchange of CO₂ between the sea and the atmosphere was studied in the Archipelago Sea, Baltic Sea, in 2017–2021, using an eddy covariance technique. The sea acted as a net source of CO₂ with an average yearly emission of 27.1 gC m^-2 yr^-1, indicating that the marine ecosystem respired carbon that originated elsewhere. The yearly CO₂ emission varied between 18.2–39.2  gC m^-2 yr^-1, mostly due to the yearly variation of ecosystem carbon uptake.