<p>The penis stylet, a sclerotized structure associated with the penis, has evolved in various animal groups, including flatworms and sea slugs. Although most often a simple, pointed tube, more complex morphologies are observed in certain taxa. Polyclad flatworms of the family Didangiidae possess a unique penis stylet with two proximally fused, pointed tubes, but its function is poorly understood. Here, we establish a new species, <i>Didangia unguis </i><b>sp. nov.</b>, collected by dredging in Tateyama, Chiba, Japan. <i>Didangia unguis</i><b> sp. nov.</b> is characterized by <i>i</i>) a strongly developed muscle bulb attached to the proximal plate of the penis stylet and <i>ii</i>) the completely independent paths between prostatic ducts from the accessory prostatic vesicles and the ejaculatory duct. Our morphological examination of the male copulatory apparatus revealed two distinct pathways within the stylet, one leading from the accessory prostatic vesicles and the other from the ejaculatory duct (which includes the seminal vesicle and the interpolated prostatic vesicle), suggesting that the penis stylet separately transfers secretions from these different prostatic vesicle types during copulation. Phylogenetic analyses using concatenated partial 18S and 28S rDNA sequences support the placement of Didangiidae within Leptoplanoidea based on the position of <i>D. unguis </i><b>sp. nov.</b></p><p><a href="http://www.zoobank.org/urn:lsid:zoobank.org:pub:58C2C315-0A3E-43EC-AC00-D76140E5338A">http://www.zoobank.org/urn:lsid:zoobank.org:pub:58C2C315-0A3E-43EC-AC00-D76140E5338A</a></p>