A new dinoflagellate species, Azadinium poporum sp. nov., was isolated as three clones from the southern North Sea off the Danish coast. In contrast to the type species A. spinosum, a known producer of azaspiracid (AZA) toxins, the isolates of A. poporum produce no known AZA analogues detectable by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Azadinium poporum is a small (11–16 mm length; 8–12 mm width) photosynthetic dinoflagellate with a thin theca exhibiting the Kofoidean plate tabulation: Po, cp, X, 40, 3a, 600. 6C, 5?S, 6000, 200 00. This species is morphologically distinguished from the type A. spinosum by a slightly lower mean cell length/width ratio, consistent absence of an antapical spine, the presence of more than one stalked pyrenoid, and the conspicuous arrangement of the ventral pore, located at the junction of the pore plate and the first two apical plates. This latter feature also distinguishes A. poporum from A. obesum. As in A. spinosum, but differing from A. obesum, the first precingular (100) plate of A. poporum touches the first epithecal intercalary plate 1a. DNA sequences and phylogenetic analysis of four nuclear-encoded genes, namely the internal transcribed spacer (ITS), 18S rDNA, 28S rDNA (D1/D2) and cytochrome oxidase I (COI), support the separation of A. poporum from (but with a close affinity to) other Azadinium species (A. obesum and A. spinosum), as well as its description as a distinct species within the recently erected genus Azadinium Elbra¨ chter & Tillmann. Nevertheless, in spite of the phylogenetic consistency at the generic level, the position of the genus with respect to higher taxonomic levels within the subclass Peridiniphycidae could not be further clarified.