The cestode fauna of the shortfin devilray, Mobula kuhlii (Müller & Henle, 1841) was examined for the first time. The work resulted in the discovery of a new genus and species of rhinebothriidean tapeworm. Crassuseptum pietrafacei, n. gen. n. sp. is erected here on the basis of its unique scolex and proglottid morphology. Histological sections and examination by light and scanning electron microscopy demonstrated that this new genus differs from all other rhinebothriidean genera in its possession of bothridia in which the proximal and distal sides are confluent, i.e., not separated by a rim of tissue, and in its possession of testes that extend to the posterior margin of the ovary. This new species is characterized in part by its possession of stalked, elongate bothridia lacking lateral constrictions, with 13–15 prominent transverse bothridial septa and 4 reduced transverse septa, craspedote proglottids, each with 2–3 layers of testes in cross section, and a vas deferens that joins the cirrus sac at its anterior margin. Histological and optical sections through bothridial septa revealed that the transverse septa are formed by septal muscles separate from bothridial radial musculature, extending from the anterior side to the posterior side of each septum. This is only the second species rhinebothriidean cestode reported from mobulids. This study adds to the number of new species and genera of elasmobranch cestodes discovered off of the island of Borneo.