Morphological and phylogenetic analyses and sequencing of the partial LSU gene and internal transcribed spacer (ITS) regions of the rDNA gene were combined to study toxic strains of Alexandrium tamiyavanichi Balech collected from northeastern Brazilian coastal waters. All specimens were identified with A. tamiyavanichi mainly based on the shape of the anterior sulcal plate (S.a.), which was the most conservative character. Among the specimens studied, 8% did not conform to the morphological description of A. tamiyavanichi. The occurrence of transitory states between A. tamiyavanichi and Alexandrium cohorticula (Balech) Balech tended to confirm that both species are conspecific. The posterior sulcal plate (S.p.) was not a good taxonomic character as variability in its shape did not allow a clear assignment of specimens from the same clonal isolate to either morphologically defined species. Phylogenetic analyses confirmed the overall validity of morphological characters to delineate the evolutionary relationships among the clades of Alexandrium species analyzed here, indicating that A. tamiyavanchi is a valid species according to the morphological species concept. This analysis showed that the Brazilian strains form a monophyletic clade with Asiatic strains of A. tamiyavanichi, but with enough genetic distance to argue for long-term separation and isolation of locally established populations, extending the known biogeographic range of this species. On the higher phylogenetic level, our data also indicate that Alexandrium tropicale Balech forms part of the clade otherwise consisting of the morphologically defined freterculus group, and this A. tamiyavanichi/tropicale/fraterculus (Balech) Balech clade (including species from the formerly termed tropical Asian [TROP] clade) may be considered as a sister group of the Alexandrium tamarense species complex.