Hamaticyclops ahni is described as a new genus and species from the Californian shore of the United States. It was discovered as a victim of the sea anemone Anthopleura elegantissima (Brandt). The new genus is closely related to the genus Hemicyclops but possesses, as significant autapomorphies, three strong claws on the second endopodal segment of antenna and a markedly elongate exopod of leg 5 with its protopod fused to somite.