Dicyemids (phylum Dicyemida) are endosymbionts that typically are found in the renal sac of benthic cephalopod molluscs. The dicyemid bodies consist of only 8 to 40 cells, which are the fewest in number of cells in metazoans, and are organized very simply. Typically, two or three dicyemid species are found in a single specimen of the host, and most of them show high host specificity. Dicyemid species have been examined in the benthic cephalopods collected from the Kumano Sea at a seafloor depth of 150–400 m using bottom trawl nets since 2013. Twenty-five undescribed species of dicyemids, included in five genera, were found in 15 species of cephalopods. There is a considerable diversity of dicyemids in relatively narrow range of localities of the Kumano Sea. Here current taxonomic studies on dicyemid fauna in the Kumano Sea are briefly reported.