Abstract Animal physiology and ecology are affected by increasing environmental temperatures, and this is particularly relevant in the tropics, where organisms are already living on the warm edge of their thermal windows. Here, we present data on sub-lethal effects of temperature (using molecular biomarkers), thermal tolerance, warming safety margins and body size shifts of a gastropod (Stramonita haemastoma) from tropical rocky shores, under an experimental setup of a climate warming scenario. Heat shock response, protein damage, antioxidant activity and lipid damage were all evaluated once a week during one month of exposure at a control temperature, and at an experimental temperature of plus 3 °C. Significant increase of heat shock protein response, lipid peroxides and catalase at the elevated temperature suggest the activation of cytoprotective pathways as response to an increased thermal load. Duration of exposure also had a significant influence in the animals’ responses, since whole body thermal tolerance only showed acclimation potential in the short-term, but not in the long-term. Thermal safety margin was low for this species, suggesting a narrow ability to tolerate further warming. Smaller body sizes were observed in specimens exposed to increased temperature, suggesting the occurrence of slower growth and possible changes in energy metabolism. Hence, enduring thermal stress, as predicted if present day warming trends are not reversed, may compromise populations of tropical marine snails.