<p>We describe a new species of polyclad flatworm, Prosthiostomum torquatum sp. nov., from the rocky intertidal zone in Shirahama, on the Pacific coast of middle Honshu, Japan. Prosthiostomum torquatum is characterised by a unique dorsal colouration, which is comprised of i) numerous orange maculae and blue dots dispersed all over the dorsal surface, ii) a transverse dark-brown line in the anterior part of the body running slightly anterior to another transverse white line, both of which are slightly bent backwards at mid-point, and iii) an incomplete, mesh-like, median band made by dark-brown pigments, fading away posteriorly. By this dorsal colour pattern, the new species can be distinguished from similar congeners such as P. trilineatum Yeri and Kaburaki, 1920 and P. komaii Kato, 1944. We performed a molecular phylogenetic analysis based on 462-bp partial mitochondrial cytochrome c oxidase subunit I (COI) sequences of four species of Prosthiostomidae currently available in public databases in addition to that of P. torquatum. In the resulting tree, P. torquatum was sister to Lurymare clavocapitata Marquina, Aguado, and Noreña, 2015 originally described from Lizard Island, Australia. While these two share a similar dorsal colouration, P. torquatum can be distinguished from L. clavocapitata by i) the absence of a common muscle bulb/sheath enclosing the whole male copulatory apparatus, ii) the median mesh-like band comprised of dark-brown pigments, and iii) COI uncorrected p-distance being 0.094. As a result, our phylogenetic tree indicates the possibility that Prosthiostomum and Lurymare as currently diagnosed may not be monophyletic, and that the common muscle bulb enclosing the whole male copulatory apparatus may not be appropriate as a diagnostic character for Lurymare.</p>