A new species of ascaridoid nematode, Hysterothylacium gibsoni sp. nov., is described based on specimens collected from the intestine of the slender lizardfish Saurida elongata (Temminck et Schlegel) (Aulopiformes: Synodontidae) in the Yellow Sea, China. The new species differs from its congeners by its small body size (12.8-13.2 mm), the absence of cervical alae, a very short intestinal caecum (representing 8.86-9.52% of oesophageal length) and a long ventricular appendix (intestinal caecum to ventricular appendix ratio 1:15.3-20.0), short spicules (0.38-0.41 mm, representing 2.97-3.11% of body length), the number and arrangement of the caudal papillae (25-28 pairs arranged as follows: 18-22 pairs precloacal, 3 pairs paracloacal, and 3-4 pairs postcloacal). In addition, Hysterothylacium tetrapteri (Bruce et Cannon, 1989) is also redescribed based on the material collected from the striped marlin Kajikia audax (Philippi) (Perciformes: Istiophoridae) in the South China Sea.