The marine phytal zone has received little attention as yet as a habitat of associated and symbiotic crustaceans, particularly copepods. We describe here the female and male of a new harpacticoid copepod species Zaus wonchoelleei from the green alga Ulva sp. attached to aquaculture floats at Tongyeong Marine Living Resources Research and Conservation Center, Gyeongsangnamdo, Korea. The new species is closely related to Z. unisetosus recorded from Japan and Korea and Z. goodsiri by having one inner seta on leg 2 endopod but can be easily distinguished by the combination of the following characters: leg 2 enp-3 with only 1 inner seta, setation of leg 5 exopod, segmentation of antennule, and sexual dimorphism of leg 2 endopod. The detailed taxonomic description of the new species, Z. wonchoelleei, addresses the similarity and/or distinctiveness of species in the genus Zaus and contributes to the discussion of characters found useful to justify the separation of closely related species reported or being documented elsewhere.