Climate change and microplastic pollution are emerging environmental stressors that increasingly threaten marine ecosystems and filter-feeding organisms. The Mediterranean mussel, Mytilus galloprovincialis, is particularly vulnerable due to its ecological and economic importance. This study explored how elevated seawater temperature and microplastic exposure alone or/in combination affect the expression of key antioxidant and stress-related genes: catalase (cat), superoxide dismutase 1 (sod1), superoxide dismutase 2 (sod2), and heat shock protein 70 (hsp70). Mussels (27.04 ± 6.45 g; 4.02 ± 0.21 cm) were assigned to four treatments: (1) control (18 °C, synthetic seawater), (2) elevated temperature (24 °C), (3) microplastics (0.06 mg L⁻¹), and (4) combined elevated temperature and microplastics. After 21 days, gill and digestive gland tissues were analyzed using quantitative PCR (qPCR). Exposure to higher temperature caused a marked increase in hsp70 expression and moderate upregulation of antioxidant genes, suggesting the activation of protective mechanisms against oxidative and proteotoxic stress. Microplastic exposure alone triggered a milder antioxidant response, consistent with particle-induced oxidative imbalance. The combined treatment produced the strongest transcriptional activation, particularly in sod2 and hsp70, indicating synergistic effects between thermal and microplastic stressors. Overall, the findings suggest that climate-driven warming can intensify the harmful effects of microplastic pollution, compromising the mussel’s oxidative defense capacity. Understanding such multi-stressor interactions is essential for predicting the resilience of marine bivalves under future ocean conditions.