A new copepod species, Lepeophtheirus schaadti n. sp., is established based on female and male specimens obtained from the Giant Kelpfish, Heterostichus rostratus Girard, 1854, and Striped Kelpfish, Gibbonsia metzi Hubbs, 1927, captured at Inner Cabrillo Beach in southern California, U.S.A. In addition, comparisons with copepod specimens identified by Wilson (1935) as L. parviventris Wilson, 1905 from the Spotted Kelpfish, Gibbonsia elegans (Cooper, 1864), in Newport Bay, California, revealed they are conspecific with L. schaadti n. sp. The new species differs from its congeners by a combination of characters that include: female with a genital complex that is more than half the length of the cephalothoracic shield and with posterolateral lobes, an abdomen that is composed of one somite and is less than one-quarter the length of the genital complex, a maxillulary dentiform process bearing a thin ridge on the inner tine and lacking a basal knob, no myxal process on the maxilliped, apically rounded tines on the sternal furca, the spine on the first exopodal segment of leg 3 inserted distally on the basal swelling, a 3-segmented leg 4 exopod, and a broad inner lobe of leg 5 that does not extend beyond the posterior margin of the genital complex; and male with three accessory claws on the antennal endopod and no myxal process on the maxilliped. L. schaadti n. sp. represents the first account of an ectoparasitic species from the Striped Kelpfish and Spotted Kelpfish, as well as the fourth ectoparasitic species reported from the Giant Kelpfish.