Boreal coastal seas are reported to be subject of high carbon load from land, yet actual published results on the trends in carbon concentration in coastal waters are scarce. We examined a unique time series of total organic carbon (TOC) concentration at 20 sites along the Finnish coast of the northern Baltic Sea to identify linkages between TOC and other key environmental parameters. Over the last 27 years, TOC concentration has increased in the northernmost Bothnian Bay and Quark sub-basins and in parts of the eutrophic southern Gulf of Finland sub-basin. In the entire coast, the average TOC concentration varied between 3 mg l−1 (at southwestern sites) and 6 mg l−1 (at northern and eastern sites), despite the fact that average TOC concentrations in the adjacent rivers were up to fourfold greater than in the seawater. Coastal and adjacent riverine TOC concentration correlated positively demonstrating the effect of terrestrial input, yet salinity-based mixing analysis proposes effective carbon removal at low salinities. TOC correlated negatively with salinity, which in turn decreased over the study period at 10 coastal sites. Total P increased in the southwestern mosaic-like Archipelago and Åland Sea sub-basin, but decreased in the northern sub-basins. TOC was not shown to be a major contributor to water clarity, but instead Secchi depth was observed to correlate frequently to Fe and chlorophyll a.