An outbreak of the fish-ectoparasitic marine isopod Caecognathia coralliophila (Monod, 1926) occurred in a fish hatchery in Sabah on Borneo Island, Malaysia. Larval stages of gnathiid isopods are parasitic on fish whereas their adult stages are free-living. The original description of C. coralliophila (Monod, 1926) was based on the morphology of one adult male from Thailand, and no description of the adult female or larva has been published to date. We compared the holotype of C. coralliophila and redescribed adult males and adult females as well as third-stage larvae from the fish hatchery. This paper provides a redescription of C. coralliophila based on all phases of the life history. Caecognathia coralliophila is distinguished from congeners by having reduced setae on the distal margins of the pleopodal endopods in adult males, biarticulate pylopods and the setae on the pleopodal rami being reduced in adult females, and antenna much longer than antennule in third-stage larvae.