Trebius shiinoi n. sp. is described from females and males collected from the uterine linings and on embryos within the uteri of 2 near-term Japanese angelsharks (Squatina japonica Bleeker, 1858) captured in Suruga Bay, off central Japan, and from female specimens reported by Shiino in 1963 that were found on embryos of the clouded angelshark (Squatina nebulosa Regan, 1906) captured off Shirahama, central Japan. Shiino identified his specimens as Trebius longicaudatus Shiino, 1954. However, our comparisons between Shiino's specimens and those newly collected revealed both to represent the same species, and comparisons of these specimens to 5 syntypes of T. longicaudatus and to published information detailing other Trebius species revealed them to be a new species that differs most notably from its congeners by the enormous length of its transformed adult female's abdomen and by the presence of a distinctive nublike seta on her caudal ramus. Trebius shiinoi n. sp. is an unusual copepod because it is an endoparasite of adult female angelsharks as well as an ectoparasite of embryo angelsharks, and it is proposed that flushing of the uterine-cloacal chambers of clouded angelsharks and Japanese angelsharks may facilitate T. shiinoi infections. A redescription of T. longicaudatus is also provided.