The giant sea bass (GSB), Stereolepis gigas Ayres, 1859, is the largest teleost (exceeding 2 m in length and 200 kg in weight) and megacarnivore found in California kelp forest communities. Overfishing of GSB in the late 1920s crashed the population off California and in 1996 it was classified as an International Union for Conservation of Nature (IUCN) critically endangered species. Recently, three GSB were collected off San Onofre, California and held at the Southern California Marine Institute in San Pedro. Two of the three GSB were infected with Lepeophtheirus longipes (Siphonostomatoida; Caligidae), a poorly described species of parasitic copepod previously recorded from the GSB and purportedly on other fish hosts. In this study, a detailed redescription of the female and the first description of the male of L. longipes are provided and all records of Lepeophtheirus longipes are reviewed. The latter revealed that L. longipes is host specific to GSB. Lepeophtheirus longipes is distinguished from its congeners by a combination of female characters that includes: (1) genital complex with prominent posterolateral lobes and is about half the length of the cephalothorax and just over two times longer than the cylindrical, indistinctly 2-segmented abdomen; (2) an antennule with a small conical process on the proximal segment; (3) maxillule with an outer conical process at the base of the dentiform process; (4) sternal furca with pointed and slightly splayed tines; (5) first exopodal segment of leg 3 with a terminal spine; and (6) third exopodal segment of leg 4 with three unequal apical spines.