Spartina densiflora Brongn. is found in coastal marshes of southwest Spain, growing over sediments containing 100–4800 ppm Zn. A glasshouse experiment was designed to investigate the effect of Zn from 0 to 100 mmol·l−1 on the growth and photosynthetic apparatus of S. densiflora, by measuring relative growth rate, leaf elongation rate, number of tillers, height of tillers, chlorophyll fluorescence parameters, gas exchange and photosynthetic pigment concentrations. We also determined total ash, Zn, calcium, magnesium and phosphorus concentrations, and the C/N ratio. At 100 mmol·l−1 Zn, S. densiflora showed a 48% biomass reduction after 1 month of treatment. Long-term effects of Zn on growth of S. densiflora consisted mainly of variations in net photosynthesis. Modification of the Zn/Mg ratio was linked to a strong decrease in RuBP carboxylase (Zn was favoured in local competition with Mg, so that the affinity of RuBisCO for CO2 decreased), oxygenase activity of RuBisCO acting as a substitute for the photosynthetic function. Also, Zn had a marked overall effect on the photochemical (PSII) apparatus and the synthesis of photosynthetic pigments. However, the results indicate that S. densiflora is capable of tolerating very high and continued exposure to Zn, as this species lowers the translocation of Zn from the nutrient solution to roots and controls Zn ion transport into leaves. Therefore, S. densiflora could be useful in the phytostabilization of soils.