The first online modern organic-walled dinoflagellate cyst determination key has been launched at www.marum.de/dinocystkey.html. This key is based on easily recognisable morphological features of dinoflagellate cysts that can be observed using standard transmitted light microscopy. To date, the key includes 96 cyst species that can be found in late Quaternary marine sediments. This key is free of charge to users, and will be continuously updated and improved by the authors. For each individual species of dinoflagellate cyst, the website provides information on its defining morphological characteristics and the cyst–motile stage relationship. It gives a comparison with other morphologically similar taxa, links to publications with original cyst descriptions and outlines their modern global distribution where this information is available. All species descriptions are illustrated by line drawings showing their most distinctive characteristics, and accompanied by high-quality bright-field photomicrographs. The key is compatible with all major computing platforms (including smartphones) and software.