Chrysochromulina megacylindra is a unicellular marine haptophyte microalga in the genus Chrysochromulina, class Prymnesiophyceae, order Prymnesiales. The species epithet megacylindra — from Greek meaning large cylinder — refers to a prominent cylindrical element of the cell's scale ornamentation, distinguishing it from related species such as C. brachycylindra, whose epithet denotes a short cylinder, and C. microcylindra, with a small cylinder. This naming pattern reflects the systematic use of scale morphology to differentiate species within the genus. C. megacylindra has been recorded from Norwegian and Swedish coastal marine waters, contributing to the known diversity of Chrysochromulina in northern Atlantic environments. The species inhabits the photic zone of coastal marine systems, where it functions as a photosynthetic primary producer. Chrysochromulina cells in this size class typically range from five to twenty micrometers in greatest dimension and are counted among the nanoplankton — a size fraction particularly important in oligotrophic and post-bloom marine ecosystems. The coiling haptonema of C. megacylindra, like that of all Chrysochromulina species, likely assists in prey capture or substrate attachment, enabling mixotrophic nutrition in environments where dissolved nutrients are limiting. The species has not been assessed under IUCN criteria and is classified as Not Evaluated. Its documentation contributes to understanding the high species richness within Chrysochromulina and the role of nanoplankton diversity in northern European marine ecosystems.