A new ascaridoid nematode Hysterothylacium longilabrum sp. nov. collected from the intestine and stomach of the marine fishes Siganus fuscescens (Houttuyn) and Siganus canaliculatus (Park) (Perciformes: Siganidae) in the South China Sea is described and illustrated. The new species differs from its congeners by the unusually long lips, the very short intestinal caecum and relatively long ventricular appendix (ratio of intestinal caecum to ventricular appendix, 1:2.38-5.50), the long spicules (1.96-3.28 mm long, representing 7.42-11.4% of the body length), the number and arrangement of male caudal papillae [38-43 pairs in total, arranged as: 31-34 pairs precloacal, 1 pair of paracloacal and 4-6 pairs postcloacal (the second or fourth pair double)] and the presence of a particular medioventral precloacal papilla in the male. Molecular analyses by sequencing and comparing the internal transcribed spacer (ITS) of the ribosomal DNA of H. longilabrum sp. nov. with the closely related nematode sequences seem to support the validity of the new species based on the morphological observation. In addition, the third- and fourth-stage larvae of the new species are also exactly identified and described by analysing and comparing the ITS sequence with the adult, and the result is a substantial step toward elucidating its life cycle.