Heterosentis holospinus n. sp. is 1 of 11 species of acanthocephalans collected from 13 species of marine fish in Halong Bay, Vietnam, in 2008 and 2009. The striped eel catfish, Plotosus lineatus (Thunberg, 1787), harbored a large number of individuals of the new species. Species of Heterosentis Van Cleave, 1931, are characterized by having 2 or 3 types of proboscis hooks in 10–14 rows, and trunk spines. Of the 15 known species of Heterosentis (including the new species), H. holospinus is distinguished by having a trunk entirely covered with spines and an anterior trunk cone free of spines. Only Heterosentis overstreeti Schmidt and Paperna, 1978, has a trunk that is also entirely covered with spines; it, however, differs from H. holospinus by lacking an anterior trunk cone but having 4 giant nucleated muscle cells in the anterior trunk, and 2 basal spines per proboscis hook row. Heterosentis holospinus has no such muscle cells, and the proboscis contains 3–4 spines per row. Only 2 other species of Heterosentis have anterior trunk cones: Heterosentis septacanthus (Sita, 1949) Golvan 1969, which has a cylindrical trunk and proboscis, only anterior trunk spines, and smaller proboscis hooks, and Heterosentis plotosi Yamaguti, 1935, which has 4 nucleated giant muscle cells, only anterior trunk spines, and 4–5 spines per proboscis hook row. The characteristics of H. holospinus are described and compared with those of the other 14 species of Heterosentis. A key to the 15 species of the genus is included. The justification for the retention of the 3 subfamilies of Arhythmacanthidae no longer applies, and their deletion is proposed.