Cucullanus hainanensis sp. nov., collected from Muraenichthys gymnopterus (Bleeker) (Anguilliformes: Ophichthidae) in the South China Sea, was described using both light and scanning electron microscopy. The new species can be readily distinguished from its congeners by the large pseudobuccal capsule, the position of excretory pore and deirids, the length of spicules (0.64-0.76 mm, 5.84-6.67% of body length) and gubernaculum (0.21-0.24 mm), the number and arrangement of caudal papillae and the particular morphology of cloacal region in male. The new species was also characterized using molecular methods by sequencing and analysing the small subunit (18S) and the second internal transcribed spacer (ITS-2) of nuclear ribosomal DNA (rDNA). In addition, Cucullanus muraenesocis (Yin et Zhang, 1983) was regarded a homonym of C. muraenesocis Yamaguti, 1961, and a new name, Cucullanus wangi nom. nov. was given to it.