The novel dinoflagellate taxon we describe here as Azadinium obesum sp. nov. was isolated as clone 2E10 from the North Sea along the Scottish east coast, the same locality as for Azadinium spinosum Elbrächter et Tillmann, the type and formerly only species for this genus. In contrast to A. spinosum, a known producer of azaspiracid (AZA) toxins, the isolate of A. obesum produces no known AZA analogues detectable by liquid chromatography coupled with tandem mass spectrometry. Azadinium obesum is a small (13-18 mm length; 10-14 mm width) photosynthetic dinoflagellate with a thin theca exhibiting the Kofoidean plate tabulation: Po, cp, X, 49, 3a, 60, 6C, 5?S, 690, 200. This species is morphologically distinguished from A. spinosum by slightly larger mean cell size, consistent absence of an antapical spine, the lack of a stalked pyrenoid and several details of the plate configuration. Among these thecal features, the first precingular (10) plate of A. obesum does not touch the first epithecal intercalary plate and is four sided rather than five sided as in A. spinosum. Furthermore, in A. obesum the lower half of the first apical (19) plate is very narrow and tonguelike, and precingular plates 10 and 60 are very close together, whereas these diverge in A. spinosum. DNA sequence and phylogenetic analysis elucidates and supports the separation (but close affinity) of A. obesum and A. spinosum, aswell as the description of the former as a distinct species. Phylogenetic interpretation of the four genes analysed - internal transcribed spacer, 18S rDNA, 28S rDNA (D1/D2) and cytochrome oxidase I - further validates the recently erected genus Azadinium Elbrächer et Tillmann but does not clarify the position of the genus with respect to higher taxonomic levels within the subclass Peridiniphycidae.