Light microscopy and scanning electron microscopy were used to study Amphidinium glabrum sp. nov., a new dinoflagellate species from tidal sand flats in the North German Wadden Sea and from the surface water and the lowermost layer of first-year ice in the European Arctic. Cells are strongly flattened dorso-ventrally, with nearly parallel sides in ventral view. The distal region of the strongly displaced cingulum is not connected with the sulcus. There is no surface striation. The sulcus extends from the antapex, where it forms a large excavation, to the apex and further onto the dorsal side of the epicone. The new species is compared with two morphologically similar dinoflagellates: A. scissum Kofoid & Swezy and A. scissoides Lebour. Characters that distinguish the three taxa are: cell shape and size, shape of epicone and hypocone, cell surface, cingulum displacement, distal region of the cingulum and antapical notch.