Abstract Wetlands are very fragile systems and susceptible to being affected by human activity. S'Albufera de Mallorca Natural Park is the main wetland in Mallorca Island (Spain), and undergoes a salinization process derived from the overexploitation of adjacent aquifers, which favors marine intrusion. The objective was to evaluate the effects of salinity changes in a channel in s'Albufera de Mallorca through the analysis of biomarkers of oxidative stress in the submerged macrophyte Myriophyllum spicatum. Six different points were analysed along a channel characterized by different salinity values, ranging from ~2 to ~11. The % of macrophyte coverage in the studied stations followed an inverse salinity pattern with greater coverage in areas of low salinity and minimal coverage in those with higher saline concentration. The photosynthetic pigments index (D430/D665), determined in M. spicatum leaves, was significantly higher at the stations most affected by salinity. All antioxidant enzyme activities – catalase, superoxide dismutase, glutathione peroxidase and glutathione reductase – followed a similar pattern of response with an increase in activity as salinity increases. Similarly, the levels of malondialdehyde, as marker of lipid peroxidation, were also increased the stations with the highest salinity. In conclusion, marine water intrusion causes an increase in the salinity of inland waters in s’Albufera de Mallorca, which is evidenced by the increase in the antioxidant enzyme activities and oxidative damage.