A new species of dinoflagellate, Protoperidinium hormusum sp. nov. (Dinoflagellata, Protoperidiniaceae), was described by means of light fluorescent microscopy. It was found in the plankton of the Strait of Hormuz, which connects the Oman and Persian Gulfs, in 2015. Cells of P. hormusum are shaped like five-pointed star, symmetrical, and moderately flattened dorsoventrally; antapical horns are widely separated, relatively thin, and divergent. The epicone is conical with an apical pore. The first apical plate 1’ is “ortho” and the second anterior intercalary plate 2a is “neutra” or “hexa”. The cingulum is medial, slightly ascending (by 0.1–0.2 of its width). The sulcus is slightly extending onto the epicone, shaped like elongated oval, not reaching the antapex. Cell dimensions: 86.7–104.4 µm in length (95.7 ± 5.18 µm) and 64.9–81.6 µm in width (72.0 ± 5.22 µm).

Keywords: new species, Dinoflagellata, Dinophyta, Protoperidinium hormusum, Strait of Hormuz, Persian Gulf, Gulf of Oman

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