Based on the extensive literature on Ordovician acritarchs, biodiversity curves of marine microphytoplankton of the palaeocontinent Baltica have been compiled. The dataset is derived from more than 100 publications and includes over 600 species whose ranges can be used in diversity analysis. Stratigraphically well-constrained data from the Rapla and Männamaa boreholes, northern Estonia, are analysed separately in order to provide additional information on the Middle to Late Ordovician microphytoplankton evolution on shallow shelf settings. The total species diversity and normalised diversity curves based on range-through data show a slight decrease from the Tremadocian to Floian, which may partly be attributed to limited data available. A continuous increase in diversity from the base of the Dapingian to late Darriwilian — early Katian can be observed, with highest total diversity approaching 250 species in the Keila Regional Stage. The highest appearance rates are recorded in the Dapingian and Darriwilian. Diversity progressively diminished after the early Katian with a more pronounced decrease in the upper Ordovician Porkuni Regional Stage, corresponding to the Hirnantian. A significant faunal turnover took place in the uppermost Ordovician Pirgu and Porkuni stages, with high extinction rates as well as the appearance of many acritarch taxa that are typical of the Silurian floras. The phytoplankton diversity curves match rather well with those of several other fossil groups in Baltica, notably brachiopods and ostracods. Comparison with other palynomorphs reveals both similarities and differences. The chitinozoans show the highest diversities in the upper Darriwilian, but their later decline predates that of acritarchs. A rapid diversification of scolecodonts (jawed polychaetes) can also be observed in the upper Darriwilian. However, unlike acritarchs or chitinozoans, their diversity continues to increase until the uppermost Katian. The acritarch diversity changes are analysed and discussed in the context of palaeogeographic (northwards drifting of Baltica) and palaeoclimatologic changes (rising sea levels up to the middle part of the Upper Ordovician). The increasing diversity of the phytoplankton not only roughly correlates with the Baltic and global sea-level changes, but also with the diversification of marine invertebrate groups.