Hypersaline stress is a major stressor in semi-enclosed coastal lagoons, affecting the distribution and survival of key foundation species. In this study, we investigated how Posidonia oceanica meadows responded physiologically and morphologically to different salinity concentrations both in-situ, across a natural saline gradient occurring inside the lagoon system, and in a mesocosm experiment. Leaf water relations, organic osmolytes, photosynthesis, respiration, Chlorophyll-a fluorescence, pigments content, and leaf growth were studied in P. oceanica from three different sites within the Stagnone of Marsala lagoon, as well as after exposing P. oceanica to a salinity level of 46 psu in a 30-day mesocosm experiment. Overall, we show that P. oceanica has evolved osmolar regulatory strategies and photosynthetic plasticity, allowing these meadows to cope with large salinity fluctuations (38 – 51.45 PSU). Our findings contribute to a better understanding of seagrass ecophysiological adaptation to extreme environmental conditions, as well as the importance of these populations serving as an experimental model at the Mediterranean scale for more comprehensive forecasting and management of environmental stress in these marine foundation species in an era of rapid environmental change.