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Abstract: Secchi depth, as a proxy of water transparency, provides valuable information on the availability of light to the underwater ecosystems. Changes in water transparency have also been widely linked to eutrophication and phytoplankton biomass. This study aimed to describe the development of water transparency in the Baltic Sea through a unique century-long set of Secchi depth observations. Furthermore, the aim was to explain the role of phytoplankton in determining water transparency in these optically complex waters. Water transparency in the open Baltic Sea has decreased during the last one hundred years. The development differs between the sub-basins of the Baltic Sea. The decrease has been most profound in the north-eastern Baltic sub-basins, but apparent also in parts of the Southern and Central Baltic. In many of the northern areas the decrease has accelerated during the last decades, whereas in the Southern Baltic a recent increase was observed. The analysis of simultaneous chlorophyll a observations during the period from 1972 to 2006 revealed that during summer time, 13–17% of the light attenuation can be attributed to phytoplankton. In spring, the average proportion is between 31 and 42%, with great variation between observations. We find Secchi depth a suitable indicator of eutrophication, integrating various organic matter related features. It should always be applied with sufficient background information of the optical properties of the water mass, and complemented by other indicators. [Copyright &y& Elsevier]