The Mediterranean Sea is undergoing increasing acidification, with surface ocean acidity having risen by approximately 26% since pre-industrial times. Due to its semi-enclosed nature, the Mediterranean basin may experience intensified impacts, particularly on autotrophic organisms such as macroalgae in rocky shore habitats. Natural CO₂ vents of Ischia Island (Italy) provide a unique natural laboratory for studying the long-term effects of ocean acidification (OA) on marine ecosystems and exploring potential resilience mechanisms. This study investigates the physiological responses of the brown macroalga Dictyota dichotoma along the acidification gradient at the Ischia vents. Sampling area included three extremely low pH replicated sites (6.25), three intermediate pH sites (7.14), and three control sites (8.13), supplemented by an external control 300m away from the acidified area, to avoid the influence of pH gradient. Apart from pH, all sites shared the same depth, exposure, and substrate characteristics. Macroalgal community composition was assessed with visual estimate of substrate coverage followed by measurements of photosynthetic efficiency. Furthermore, thalli were analysed for functional traits, pigment composition and antioxidant response. Results revealed D. dichotoma as a dominant species under highly acidified conditions. D. dichotoma maintains consistent photosynthetic efficiency under low pH, while reinforcing its structural tissue, likely suggesting a strategy that aim at withstanding chronic stress. Moreover, data showed a significant upregulation of antioxidant and phenolic defences in acidified environments, highlighting a noticeable ecological plasticity. The high antioxidant pool in acidified sites has a pivotal role in enabling D. dichotoma to mitigate oxidative damage and persist in altered seascapes. Overall, these findings highlight the ecological plasticity of D. dichotoma and its adaptive capacity under OA scenarios, positioning it as a potential model for further applied research into marine resilience in high-CO₂ environments.