Abstract: This study, via biochemical and ecological methods, investigated the influence of CO₂induced acidification on the antioxidant parameters (i.e., superoxide dismutase, SOD; catalase, CAT; glutathione Stransferase, GST; and lipid peroxidation, LPO), development time and fecundity in marine copepod Tigriopus japonicus. The results showed that acidification significantly induced the antioxidant enzyme (SOD and CAT) activities, and increased LPO level in the copepod, while the GST activity was not affected, indicating that T. japonicus might increase the activities of SOD and CAT to fight against oxidative stress caused by acidification. In addition, development time for nauplius phase and development time to maturation in T. japonicus were strikingly shortened by acidification, which significantly inhibited the copepod’s fecundity. Thus, acidification could affect the population level of this copepod, i.e., population replenishment and dynamics. In summary, this study demonstrated that ocean acidification could cause oxidative stress on copepod T. japonicus, hence inducing oxidative damage to biological macromolecules (such as proteins, nucleic acids, etc.), and ultimately affecting its individual level and population dynamics (e.g., shortened development time and suppressed fecundity).