We present guidelines for using environmental DNA metabarcoding, together with conventional techniques, to monitor the taxonomic diversity of phytoplankton in marine and brackish waters. The focus is on eukaryotic and prokaryotic phytoplankton using 18S and 16S rRNA primers and high-throughput sequencing. Information and recommendations on sampling, sample processing, molecular biological work, quality control, bioinformatics, data storage and management and cost estimates are included so that the method can be used to complement standardized light microscopy. A scientific literature review, discussion on future perspectives, reference databases and standardization are included. Using eDNA metabarcoding to complement standardized light microscopy advances conventional monitoring and research on phytoplankton communities to assess biodiversity and the state of the marine environment.