A new heterotrophic dinoflagellate, Protoperidinium vietnamicum sp. nov. was described from the coastal waters of Viet Nam. It is characterized by its elongation, pentagonal to pyriform body in ventral or dorsal view, and moderately dorsoventrally compressed. Epitheca is tapering into a robust apical horn and shorter than antapical horns. Cingulum equatorial, slightly descending. Sulcus wide, shallow, and slightly oblique. Plate 1’ is ortho-type, and intercalary 2a and 3a plates are pentagonal. Plate 1a is quadrangular, and its area is approximately triple times smaller than plate 3a. A new combination is proposed, the Protoperidinium curvicorne (Böhm) comb. nov. has distinct morphological features from other members of the section Oceanica. It presents a large cell-sized species with a round, broad body, and strong dorsoventral compression. The epitheca abruptly transitions into a slender apical horn strongly tilted towards the ventral side. Cingulum is narrow, shallow, descending, and strongly inclined relative to the equatorial plane of the cell. The plate 1’ is ortho-type, and 2а and 3а are asymmetrical and pentagonal. Both species are provided with detailed descriptions and are illustrated with line drawings, as well as with light microscopy images and SEM micrographs. In-depth discussion and comparison with similar species are presented.