Ocean acidification is expected to alter carbonate chemistry in coastal waters, with important consequences for calcifying bivalves that depend on shell integrity and physiological stability for survival. In this study, adult P. viridis were exposed under controlled laboratory conditions to ambient pH (7.9), moderate acidification (pH 7.7), and low pH (7.4) to evaluate the effects of ocean acidification on survival, shell microstructure, shell surface elemental composition, and tissue organization. Survival was assessed using Kaplan–Meier analysis, shell surface morphology was examined using FESEM, elemental composition was determined by EDS, and gill and digestive gland tissues were evaluated histologically. Survival analysis revealed a treatment specific and non-linear response. Mussels maintained high survival under ambient pH and pH 7.4, whereas pH 7.7 produced a significant reduction in survival, with a median survival time of 26 days. FESEM observations showed that acidified treatments altered both inner and outer shell surfaces, producing localized cracks, surface roughening, pore-like depressions, fragmented mineral aggregates, and disorganized shell structures. EDS analysis further demonstrated significant pH-associated changes in shell surface chemistry, particularly a marked decrease in carbon and increases in oxygen and calcium at pH 7.4, accompanied by elevated O/C and Ca/C ratios. Histological examination revealed structural alteration of gill lamellae under acidified conditions and clearer digestive gland disruption at pH 7.4, including tubule deformation, enlarged luminal spaces, epithelial thinning, and atrophy-like changes. Together, these findings indicate that green mussels may maintain survival under acidified conditions while still experiencing sublethal shell and tissue stress. The integration of mortality, FESEM, EDS, and histology demonstrates that ocean acidification affects P. viridis across multiple biological levels and that survival alone may underestimate its physiological cost. This study highlights the value of multi-endpoint assessment for understanding green mussel vulnerability and resilience under future acidified coastal environments.