The damaging effect of mid-ultraviolet (UVB, 280–315 nm) radiation on a marine copepod, Tigriopus japonicus sensu lato was investigated. Copepods were collected from tidal pools on rocky shores, Yeosu, on the south coast of Korea, and were cultured under constant conditions (temperature: 20°C, salinity: 20 psu, L : D = 12 : 12 h) in the laboratory. Each stage of nauplius (N1–N6) and copepodite (CI–CVI) was harvested from the culture and exposed to one of eight irradiation doses (1, 5, 10, 15, 20, 25, 30, and 35 kJ m−2) of UVB. No deleterious effect was induced by UVA or PAR. Extensive morphological deformities were observed, i.e., shrunken body (in groups 5 kJ m−2 at N4; 1 and 15 kJ m−2 at CI), in the urosome and caudal rami (5 and 15 kJ m−2 at N6; 10 kJ m−2 at CI), in the swimming legs (5, 10, and 15 kJ m−2 at CI; 10 kJ m−2 CIV), and in the antennule (5 kJ m−2 at CIII; 1 kJ m−2 at CIV). The types of deformity observed depended on the developmental stages of copepods that were exposed to UVB radiation. UVB radiation had a damaging effect on the morphology of T. japonicus s. l. Exposure of earlier larval stages to mid-ultraviolet affected the shape of the entire body, and the prosome shape was abnormal. Exposure of later larval stages to mid-ultraviolet resulted in abnormalities at the appendage level, i.e., in the antennules and swimming legs.