Two new species of Tetrastemma genus, found on Phyllospadix iwatensis leaves 1-2 m deep, have been described. The new species are characterized by small body size of 4-8 mm with semitransparent yellowish coating, and four red eyes on the head. Subepidermal glands, retraction muscles, and closed precerebral septum are well developed. The short intestinal caecum has two pouches. T. phaeobasisae sp. nov. has a head gland with short dorsal lobe, and ventral lobe, reaching brain ventral comissure, unbranched intestinal diverticula, dark basis of a central stylet. T. dilutebasisae sp. nov. has a weakly developed head gland, branched intestinal diverticula, and light-colored basis of a central stylet. The pouches of intestinal caecum usually reach the level of the brain comissure in T. phaeobasisae and do not reach the brain in T. dilutebasisae. The constancy of the head gland size, shape of intestinal diverticula, number of pouches of the intestinal caecum and their position near the brain, has been stressed.