Water hypoxia, a condition of reduced dissolved oxygen (DO), is one of the most pervasive and increasing stressors linked to climate change, eutrophication, and anthropogenic impacts. Sessile species, such as mussels, are naturally exposed to these fluctuations in oxygen availability since their immobile lifestyle prevents escaping, and this makes them particularly vulnerable to stress. We here aimed to clarify the influence of different periods of exposure to water hypoxia on crucial physiological and biochemical responses of M. galloprovincialis. To this purpose, mussels were exposed to 4 (short) and 10 (prolonged) days of hypoxia (DO at 2.5 mg L-1) and both gills and digestive gland (DG) were used to evaluate the tissue oxidative status in terms of lipid peroxidation and protein oxidation, the expression and the activity of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT), the expression of the stress-related protein HSP70, together with cell viability, and the cell response to volume changes. We found that, under prolonged hypoxia, lipid peroxidation increased in both DG and gills, while no changes in protein oxidation were observed. In the DG, exposure to both short and prolonged hypoxia is accompanied by the activation of the antioxidant response, with both SOD activity and transcriptional levels markedly upregulated. In the gills only, SOD gene expression increased at 10 days of treatment. In contrast, CAT activity and gene expression, as well as HSP70 expression levels, remained stable across tissues and treatments. Overall, these responses point to a tissue-specific and time-dependent modulation of both oxidative status and antioxidant defenses in mussels challenged by hypoxic stress.