Several studies suggest that potential competition exists between marine cations and heavy metals for binding sites on the cell wall of mosses. This competition would impact the heavy metal concentration measured in mosses by biomonitoring programs, which may underestimate air pollution by heavy metals in a coastal environment. In the present study, we aim to identify possible mechanisms affecting lead uptake by mosses in a coastal environment, specifically, the competition between lead (Pb2+) and sodium (Na+) for binding sites in Hypnum cupressiforme (Hc). We also compared the response of continental and coastal Hc populations to Pb2+ exposure by immersing the moss samples in artificial solutions that comprised six experimental treatments and subsequently locating and quantifying Pb2+ and Na+ using the sequential elution technique and X-ray microanalyses with a scanning electron microscope. We demonstrated that high concentrations of Pb2+ prevented Na+ from binding to the cell wall. We also examined the effect of the salt acclimation of Hc on Pb2+ and Na+ accumulation. Coastal Hc populations accumulated more Na and less Pb than continental Hc populations in all treatments. Moreover, our results showed treatment effects on the intra/extracellular distribution of Na+, as well as site. This feedback on the influence of salt stress tolerance on Pb2+ uptake by mosses requires further study and can be investigated for other heavy metals, leading to a better use of mosses as biomonitoring tools.