Coastal macroalgae may be subjected to global and local environmental stressors, such as ocean acidification and heavy-metal pollution. Furthermore, the responses of macroalgae may vary depending on their growth stages. Hence, we examined the combined effects of two pCO₂ levels (400 and 1000 ppmv) and five copper concentrations (0, 0.2, 0.5, 1 and 2 µM) on the juvenile and adult sporophytes of Saccharina japonica in this study. The finding demonstrated that under 400 ppmv level, increased copper concentrations significantly reduced the growth of juvenile S. japonica, but were beneficial for photosynthetic physiology, such as promoting rETR, synthesis of photosynthetic pigments and soluble carbohydrates; the growth and photosynthetic physiology of juvenile S. japonica under different copper concentration treatments did not differ significantly under 1000 ppmv, suggesting that increased pCO₂ mitigated the detrimental effects of copper pollution on juvenile S. japonica growth. Under two pCO₂ conditions, excess copper significantly promoted the growth and photosynthetic efficiency (Fv/Fm) of adult S. japonica; the soluble carbohydrates contents were enhanced when copper concentration increased under 1000 ppmv, suggesting a synergistic effect of ocean acidification and excessive copper on growth and photosynthetic physiology of adult S. japonica; the ultrastructure of cells revealed that under low pCO₂ level, the thylakoid in adult S. japonica became loose at higher copper concentration, but this damage was mitigated at higher pCO₂ level. These results indicated that excess copper will inhibit the growth of juvenile S. japonica, but promote the growth of adult S. japonica, indicating a positive effect of elevated pCO₂ on the response of S. japonica to increased copper concentrations. Consequently, in future oceanic conditions, ocean acidification can alleviate the detrimental effects of copper stress on juvenile S. japonica and synergistically promote the growth of adult S. japonica, which is beneficial for this macroalga to withstand copper pollution during its cultivation period.