A new species of siphonostomatoid copepod, Dissonus pastinum, is described from the horn shark, Heterodontus japonicus (Dumeril), from Awa, Japan. The new species differs from all congeners except D. ruvetti Nunes-Ruivo & Fourmanoir, 1956 and D. nudiventris Kabata, 1965 by the presence of the sternal furca and absence of spines on the genital complex. However, the new species can be distinguished from D. ruvetti by having a 1-segmented abdomen and from D. nudiventris by the absence of a large accessory spine on the second antennal claw.