This paper presents the results of a comprehensive study of the Baltic Sea carbon budget. The Baltic Sea is very much influenced by terrestrial carbon input. Rivers are the largest carbon source, and their input amounts to 10.90 Tg C yr⁻¹ (Tg = 10¹² g) with a 37.5% contribution of organic carbon. On the other hand, carbon is effectively exported from the Baltic to the North Sea (7.67 Tg C yr⁻¹) and is also buried in bottom sediments (2.73 Tg C yr⁻¹). The other sources and sinks of carbon are of minor importance. The net CO₂ emission (1.05 Tg C yr⁻¹) from the Baltic to the atmosphere was calculated as the closing term of the carbon budget presented here. There is a net loss of organic carbon, which indicates that the Baltic Sea is heterotrophic.