Three sand-dwelling species of the marine dinoflagellate genus Sinophysis were examined from intertidal and subtidal North German Wadden Sea sand and are illustrated by light and scanning electron microscopy. Two are described as new: S. stenosoma sp. nov. and S. grandis sp. nov. Sinophysis stenosoma sp. nov. is compressed laterally with a cell size of 37–56 μm long and 21–33 μm wide, a length/width ratio of 1·43–1·89, an epitheca width (dorsoventrally) of 5·5–8·0 μm, and a sulcus length of about half the cell length. Sinophysis grandis sp. nov. is compressed laterally with a cell size of 50–59 μm long and 35·0–41·5 μm wide, a length/width ratio of 1·29–1·61, an epitheca width (dorsoventrally) of 12–17 μm, and a sulcus length of about three-quarters the cell length. The third species, Sinophysis ebriolum (Herdman) Balech, is the most common species in the Wadden Sea. All species occur throughout the year and are all sand-dwelling, non-photosynthetic, with a smooth theca. They can be distinguished by size, length/width ratio, epitheca size, relative sulcal length and thecal pore pattern. A key to the five known Sinophysis species is provided.