A new nematode species, Raphidascaris (Ichthyascaris) arii sp. n. (Anisakidae), is described from male and female specimens found in the intestine of two species of marine siluriform fishes, the spotted catfish Arius maculatus (Thunberg) (Ariidae) (type host) and the striped eel catfish Plotosus lineatus (Thunberg) (Plotosidae) from the coastal region of the Gulf of Thailand, Thailand. Based on light and scanning electron microscopy examinations, the new species differs from other nine representatives of the subgenus Ichthyascaris Wu, 1949 mainly in the length of spicules (210–333 μm), body length of gravid females (10–17 mm), and in the presence of small cuticular spines or protuberances on the tail tip of both sexes and 21–30 pairs of preanal and 8 pairs of postanal papillae in the male. This is the first species of this subgenus reported from fishes of the order Siluriformes and the first species of the subgenus Ichthyascaris Wu, 1949 recorded from the Gulf of Thailand.