Polylabris lingaoensis sp. n. is described from the gills of the bald glassy, Ambassis gymnocephalus (Chandidae), from the Gulf of Tonkin (South China Sea), near Lingao, Hainan Province, China. The new species is characterized by a midventral vaginal pore, comparatively few (5-7) testes, and 2 parallel rows each comprised of 30-43 microcotylid clamps in the haptor. Polylabris lingaoensis is the only member of the genus known to parasitize a chandid host. Polylabris cf. mamaevi is described from the gills of the mottled spinefoot, Siganus fuscescens (Siganidae), from the South China Sea, which represents new host and locality records for the helminth. The gill parasites from S. fuscescens are tentatively assigned to P. mamaevi pending new collections and restudy of microcotylid species from siganid fishes.