Stramonita brasiliensis is a marine intertidal gastropod, an active predator, constantly exposed to salinity variations in its intertidal rocky habitat. The goal of this study was to investigate whether the species would handle salinity increase as well as salinity decrease, given the general paucity of data on molluscs in hypersaline seawater. Snails from Santa Catarina State, southern Brazil, were exposed to abrupt (from full-strength seawater to freshwater or to 70 psu) or gradual (full-strength seawater down to freshwater or up to 70 psu) salinity challenges. The isolated muscular foot was challenged with hypo- and hyperosmotic shocks, both of 50% with respect to the isosmotic control. Behaviour (release from the substrate and operculum closure), mantle cavity water and haemolymph osmo-ionic concentrations, and tissue hydration were then assayed. When abruptly exposed, snails remained open and active in hypersaline seawater (until 55 psu), but tended to close the operculum upon seawater dilution (salinity 15 psu). In agreement, tissues swelled more in hyposmotic saline than they shrank in hyperosmotic saline. The great euryhalinity displayed by this snail may contribute to its widespread distribution on intertidal rocky coasts.