Amphidoma (Amphidomataceae) mostly attract attention because of their production potential of the lipophilic polyether phycotoxin azaspiracid (AZA). The genus Azadinium probably has a very wide geographical distribution. Blooms of Azadinium from the continental shelf off Argentina have been observed back in the early 1990, but were just recently published, and the causative species, identified at that time as Azadinium cf. spinosum, could not unequivocally be determined. Here we retrospectively analyzed old archived samples of one of the South Atlantic Azadinium bloom from 1991 with electron microscopy. It turned out that the dominant nanoplanktonic dinophycean species in fact represent a new species which we describe here based on the morphology. Azadinium luciferelloides sp. nov. is a small (approximately 9–14 μm cell length) thecate dinoflagellate with the dominant plate pattern of the genus (Po, X, 4´, 3a, 6´´ , 6C, 5S, 6´´´, 2´´´´), and with a small antapical spine. Azadinium luciferelloides differed from all other described species of Azadinium by the position of the ventral pore, which was located on the right ventral side in a notch of an otherwise symmetric pore plate. In addition, we recorded and documented the presence of other similar sized species of the Amphidomataceae in the samples. Our finding of Az. spinosum, Az. dalianense, Az. dexteroporum, and Amphidoma languida are the first record for the South Atlantic and thus describe an important range extension of these species. The diversity and importance of the Amphidomataceae for South Atlantic spring bloom plankton is now known and taxonomically documented, but cultures and/or analysis of AZA in field samples of the area are needed to clarify the AZA production potential of the local species and populations in order to finally evaluate the risk potential of AZA for AZA shellfish contamination in the Southwestern Atlantic region.