﻿AbstractSymbiotic associations between zoantharians and sponges can be divided into two groups: those that associate withDemospongiaeand those that associate withHexactinellida.ParachurabanashinseimaruaeKise,gen. nov. et sp. nov., a new genus and a new species ofHexactinellida-associated zoantharian from Japanese waters, is described. It is characterized by a combination of the following: i) its host hexactinellid sponge, ii) very flat polyps, iii) cteniform endodermal marginal muscles, and iv) characteristic mutations in three mitochondrial regions (including a unique 26-bp deletion in 16S ribosomal DNA) and three nuclear regions.ParachurabanashinseimaruaeKise,gen. nov. et sp. nov.is the third genus in the familyParazoanthidaethat is reported to be associated withHexasterophorasponges. Although specimens have so far only been collected on Takuyo-Daigo Seamount off Minami-Torishima Island in Japan, unidentified zoantharians of similar description have been reported from the waters around Australia, indicating that the species might be widespread across the Pacific.