A new species of harpacticoid copepod Alteuthoides affinis associated with the sponge Callyspongia elegans (Thiele) was collected from offshore waters of Cheju Island, Korea, and is described with illustrations. In this genus, only one species, A. kootare Hicks, has been known to occur in association with an hexacnellid sponge from New Zealand. The new species is similar to A. kootare in the general morphologies such as the shape of body, oral appendages, and thoraxic legs. However, it can be separated from A. kootare by the expanded genital double semite, P1 exopod bearing only one claw without reduced accessary nail, and acute rostrum.