The European flat oyster (Ostrea edulis) is a foundational ecosystem engineer that has suffered widespread population declines due to overfishing, habitat degradation, and disease. Effective restoration requires understanding how environmental stressors impact juvenile oysters, a critical life stage for survival and recruitment. While trace metals such as zinc (Zn) and copper (Cu) are essential micronutrients, elevated concentrations can disrupt cellular physiology and immune function, particularly under temperature fluctuations associated with climate change. In this study, we investigated the combined effects of dissolved Zn (100 and 1000 μg L^-1) and Cu (10 and 100 μg L^-1) at three temperatures (5 °C, 15 °C, 22 °C) on immune defenses in juvenile O. edulis. Baseline tissue concentrations in controls were 73.0 ± 9.5 μg g^-1 Cu and 1240.5 ± 113.7 μg g^-1 Zn and were unaffected by temperature. Low metal exposures did not alter tissue concentrations, whereas high exposures induced temperature-dependent accumulation, peaking at 22 °C (CuH: 381.9 ± 78.6 μg g^-1; ZnH: 4573.6 ± 603.8 μg g^-1). Temperature strongly modulated cellular immunity: hemocyte abundance was highest at 15 °C, phagocytosis and acid phosphatase activity increased at 22 °C, while lipid peroxidation levels were elevated at 5 °C and 22 °C, indicating stress-induced responses at thermal extremes. Zn stimulated acid phosphatase but suppressed phenoloxidase activity, whereas Cu increased hemocyte mortality and modestly stimulated phenoloxidase. Lysozyme activity was elevated at cold exposure (5 °C), suggesting enhanced antibacterial defenses. Multivariate PLS-DA analyses revealed clearer separation of immune profiles by temperature than by metal exposure. These results demonstrate that temperature is the dominant factor shaping immune competence in juvenile O. edulis, while metal contamination at environmentally realistic levels exerts only minor effects, with implications for restoration and management under climate change.