Cerapus longicervicum, a new species of ischyrocerid amphipods, was isolated from the vicinal sea of Deokjeokdo-Island, Incheon (South Korea). This species is characterized by a narrower coxal plate width of gnathopod 1 in male, which is 0.39 times as broad as pereonite 1, and antenna 2, which is longer than antenna 1. This paper also provides the partial sequence of the mitochondrial cytochrome c oxidase subunit 1 (CO1) of the new species, which was obtained from a specimen that was preserved in 4% formalin solution.