The dinoflagellate subfamily Diplopsalidoideae encompasses 11 genera whose plate patterns show a large diversity. In a recently published molecular phylogeny (Liu et al. 2015) some of these genera (e.g. Diplopsalis, Diplopelta) are polyphyletic, suggesting that further subdivision of these genera is needed. Here we established the cyst-theca relationship of Diplopsalis caspica by incubating cysts collected from the East China Sea. Cells of D. caspica display a plate formula of Po, X, 3′, 1a, 6″, 3c+t, ?4s, 5″′, 1″″, characterized by a small, parallelogrammic anterior intercalary plate (1a) located in the middle of the dorsal part of the epitheca. The cysts are spherical and smooth-walled with a theropylic archeopyle. In addition, we obtained four large subunit ribosomal DNA (LSU rDNA) sequences from the germinated motile cells by single-cell polymerase chain reaction. Strains of D. caspica from the marine environment of the East China Sea differ at 0–2 positions of LSU rDNA sequences from that of lacustrine strains from NE China. In the molecular phylogeny, D. caspica was close to Lebouraia pusilla but distant from D. lenticula, the type species of Diplopsalis. Our results support the systematic importance of plate 1a, and therefore D. caspica was transferred to a new genus, Huia. The conservative LSU rDNA sequences in H. caspica suggest that the marine-freshwater transition occurred recently.