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A new heterotrophic dinoflagellate, Protoperidinium vietnamicum sp. nov. is described from the coastal waters of Vietnam. It is characterized by an elongated, pentagonal to pyriform body in ventral and dorsal view, with moderate dorsoventral compression. Epitheca is tapering into a robust apical horn, which is shorter than the antapical horns. Cingulum is equatorial, descending by about 0.50–0.75 of its width. Sulcus is wide, shallow, and slightly oblique. Plate 1′ is ortho‐type, and intercalary 2a and 3a plates are pentagonal. Plate 1a is quadrangular, having approximately one third the area of plate 3a. Additionally, the new combination Protoperidinium curvicorne (Böhm) comb. nov. is proposed. It is a large cell‐sized species with a round, broad body, and strong dorsoventral compression. Its epitheca narrows abruptly into a slender apical horn strongly tilted towards the ventral side. The cingulum is narrow, shallow, descending by about 1.5–2.0 cingular widths, and strongly inclined relative to the equatorial plane of the cell. The plate 1′ is ortho‐type, and 2a and 3a are pentagonal. Specimens from the type locality of the taxon were examined. Both species are provided with detailed descriptions and are illustrated with line drawings, as well as with light microscopy images and SEM micrographs. An in‐depth discussion and comparison with similar species are presented. [ABSTRACT FROM AUTHOR]