Long term measurements of phytoplankton composition in the Dutch national part of the North Sea. Samples were taken 12 - 24 times per year between 2000 and 2018 and analysed to the level of genus, species, or other taxon group. The dataset has been used for assessment of the marine environment and habitat quality in the scope of the OSPAR Quality Status Reports and the Marine Strategy Framework Directive. Currently, there are known issues concerning the quality of the data caused by changes in analytic procedures and labs. When more information is available, this will be published through EMODnet. The monitoring is directed and the dataset is maintained by Rijkswaterstaat, the executing body of the Dutch Ministery for Infrastructure and Environment. Publication of the dataset has been facilitated by Deltares, The Netherlands.